A section of a leaf of Geranium arboreum shows that it has the thinnest leaves of the Hawaiian geraniums. Thicker leaves are generally indicative of dryer conditions. The leaves of Geranium arboreum have no hypodermis, and palisade parenchyma is confined to the adaxial surface. It has the most mesomorphic leaves of the Hawaiian geraniums (M = 392).
The leaves of Geranium cuneatum hypoleucum are arranged more nearly vertically than horizontally, thereby avoiding facing sunshine during the hottest part of the day. The lower surfaces are bright with reflective hairs. This plant was photographed on Mauna Loa.
A section of the leaf of G. cuneatum hypoleucum indicates features very similar to those of G. cuneatum cuneatum: relatively great thickness, palisade parenchyma on both surfaces, and presence of a layer of hypodermis. (M = 3.92)
In sectional view, the leaves of Geranium humile are relatively thin compared with those of the Geranium species from dry sites on Hawaiian mountains. However, the presence of hypodermis in leaves of G. humile and other features suggest a degree of adaptation to bright sun and heat that can occur occasionally in the mountain bogs. Palisade parenchyma is present on only one surface of the leaves. (M = 9.14)
In sectional view, the leaves of Geranium tridens are the thickest found in Hawaiian geraniums. They have hypodermis as well as stiff hairs with grooved surfaces on both surfaces. Palisade parenchyma and stomata occur on both sides of the leaf. The outer walls of the epidermal cells on both surfaces are notably thick. Its leaves are the most xeromorphic of the Hawaiian geraniums (M = 1.22).
The leaves of Tittmannia esterhuyseniae (Bruniaceae) are flattened and have hairs along the margins. The leaves are upwardly appressed, and have stomata mostly on the adaxial surface rather than the exposed abaxial surface.
Stomata are confined to the lower surface of the needle-like leaves of Linconia alopecuroides (Bruniaceae). The epidermis is smooth in this stomatal zone, whereas the epidermal cells are lobed elsewhere, and interspersed with slender, stiff, nonglandular trichomes.
The epidermal cells of leaves of Nebelia sphaerocephala (Bruniaceae) are rounded and have ridges on them. Ridges on leaves may related to dissipating the heat load. The small size of leaves in most Bruniaceae also minimizes heat loading.
The leaf tip of Brunia albiflora—and all Bruniaceae—consists of a structure the nature of which is not clear. Leaf teeth are often assumed to be hydathodes, and in that, they may have that function in younger leaves of Bruniaceae, although stomata, characteristically associated with hydathodes, have not yet been observed in the terminal teeth of leaves of Bruniaceae.
The terminal tooth of a Linconia cuspidata (Bruniaceae) leaf looks eroded. If one studies the terminal leaf tooth of mature leaves of Bruniaceae, one finds that a periderm underlies the tooth, and thus cork layers close off the terminus of the leaf.
Comparative plant anatomy: how wide a net ? When I began to study comparative plant anatomy, I had the ideal that someone in that field could ideally study all aspects of plant structure. I tried to do that in my thesis on Fitchia, published in 1957. I tried there to connect Fitchia to composites related to it by means of studies in leaf and wood anatomy, by means of floral structure, and even by means of pollen structure (embryology was reportedly uniform within the family). With the availability of TEM and SEM, study of pollen structure soon became a time-intensive activity, so those involved did not also work in vegetative anatomy or in floral anatomy and development. In my younger days, when light microscopy was the sole tool used in palynology, studies in pollen microstructure seemed feasible, and I did some, in Rapateaceae, Guayana mutisioids, and, most notably, the fascinating and complex pollen tetrads of Sarcolaenaceae [ PDF ].
Although my thesis on Fitchia was an attempt to find relationships of that strange genus within Asteraceae, I incorporated a revision of the genus in the thesis and its 1957 published version, and leaf anatomy played a role in that, because the species did differ from each other in leaf structure.
Studying leaf venation and transactional leaf anatomy seemed easily accomplished activities, and I continued them in my studies of Centaurodendron, Hesperomannia, and the Guyana mutisioids. The most satisfying leaf anatomy studies, however, are those that connect structure with ecology and physiology. To do such studies, one must see the plants in the field, and collect and preserve them as part of the field work.
Leaf anatomy to show adaptive radiation. In 1953, when I first visited the Hawaiian Islands, I was delighted to see the Haleakala silversword, Argyroxiphium sandwicense subsp. macrocephalum. Sitting on a cinder cone where it grew, I noticed the thickness of its leaves, and like an inquisitive kid, I broke one open. I was surprised when I squeezed the broken surface and saw that a gel oozed out. This was no ordinary succulent! I pickled materials and discovered that the leaves did contain intercellular deposits of pectic compounds, which I theorized must form a water reserve, a system alternative to the tank-cell system that most succulents have [ PDF ]. Such gels also occur in leaves of mainland Madinae, such as Madia [ PDF ]. The condensed form of the leaves of upright stems of continental Madinae have condensed leaves [ PDF ]; [ PDF ]. These condensed leaves differ in anatomy from the rather mesomorphic leaves of the rosette, but these latter leaves are like those of the vernal tarweeds, which flower before the hot summer months, during which Calycadenia and Holocarpha flower. Meanwhile, the Hawaiian genus Dubautia shows leaf structure related to the remarkable adaptation radiation in that genus [ PDF ]. The leaf anatomy of the tarweeds and especially the glandular hairs on them [ PDF ]show diversity that can be related to the taxonomic system, but in turn, these characteristics are closely keyed to the ecology and habits of the genera and species. See Tarweeds and Silverswords in this website
My field work in the Hawaiian Islands went well beyond the Madiinae, both because I was interested in the question of secondary woodiness on islands, and because I had decided to write a natural history of the Hawaiian Islands. To accomplish those projects, I collected and preserved (both pickled and dried) material of various Hawaiian genera. And I photographed as much of the native flora as I reasonably could, at the same time. The Hawaiian species of Geranium seemed like a fascinating project for a study in leaf anatomy, and it was [ PDF ]. As in many Hawaiian genera, the Hawaiian species of Geranium seemed clearly the product of a single original colonization that has diversified into varied habitats. The leaves seemed clearly to relate of that ecological diversification. How to describe this? One can describe this in qualitative terms, of course. But I thought that in addition, some quantitative indices of advancement from mesomorphy into xeromorphy could be devised.
So I devised the index:
M = --------------
(E1 + E2) L
in which M = an index of mesomorphy (the higher the value, the more mesomorphic), A = the leaf area in mm2, T = the number of leaf teeth per leaf, E1 = the thickness of the outer wall on the adaxial epidermis in µm, E2 = the thickness of the outer wall of the abaxial epidermis in µm, and L = the thickness of the leaf, in µm. Results for various taxa are indicated below pictures of the leaf sections here.
As one can see, the values have a wide range. The highest are for G. arboreum, which grows in gulches at about 600 m on Haleakala, and least for G. tridens (perhaps better called G. cuneatum subsp. tridens), on alpine Haleakala. The relatively low mesomorphy value for the species in the bogs of Puu Kukui, Maui, G. humile, is perhaps understandable if one takes into account that sunny and hot hours can occur in the Puu Kukui bogs.
The index, a rather pretentious one perhaps, should not be expected to work for any particular genus—for example, genera in which drought deciduousness of leaves occurs would not operate according to the above features. Today, a good student of plant physiology would want to know the transpiration characteristics of the leaf, and such things as diffusive resistance of the leaf. Hopefully, there can be more botanists who wish to combine knowledge of leaf physiology with knowledge of leaf anatomy.
One fascinating feature in the leaves of Hawaiian species of Geranium is the occurrence of ridges on the surface of the trichomes and on the epidermal walls (not present on upper epidermis except for G. tridens). These are not on a cuticle, because the Hawaiian Geranium species, although some have very thick walls, do not have a distinct cuticle layer. The ridges also occur on the nonglandular trichomes. These trichomes give a whitish or even silvery look to the leaves. Are the ridges involved in this appearance? Probably yes, such ridges also occur on nonglandular trichomes of the leaves of Argyroxiphium and Raillardella, and may be found around stomata on leaves of various plants. Are they effective in radiating heat from the leaf? Where most abundant, as in Argyroxiphium, they may be effective in reflecting UV light.
Leaf anatomy as an indicator of the taxonomic system. Although I was interested in wood anatomy of Bruniaceae, and collected woods of that family in South Africa in 1991, I studied the transectional and surface leaf anatomy of the family also. Why not? I collected them. The thick, tough epidermal walls of leaves of Bruniaceae do not collapse when the leaves are dried, so that leaves from herbarium specimens are as good for SEM studies as are pickled or critical-point-dried leaves. The leaf anatomy of Bruniaceae, not surprisingly, showed some differences among the species [ PDF ]. When I constructed an index to indicate relative mesomorphy or xeromorphy of the leaves, the results showed that leaves of all Bruniaceae are rather xeromorphic. Is that a negative finding, compared with the rather spectacular radiation in leaf anatomy in the Hawaiian tarweeds and geraniums? Not at all. What it shows is that Bruniaceae are adapted to the seasonally hot and dry conditions in the fynbos of Cape Province, South Africa. The word fynbos is Afrikaans for “fine bush,” and connotes the slender (and thereby xeromorphic) leaves so typical of many species on the sandstones of Cape Province. The flora of Cape Province is essentially a xeromorphic flora, with pockets of mesomorphic plants in localized areas where wetter conditions prevail.
Leaf anatomy as an indicator of hybridization. At Pomona College, where I taught for 37 years, the production of a senior thesis was a graduation requirement for students. I thought that students shouldn’t play at research, they should do research. The students who did senior theses with me emerged with a publication. One such senior-thesis-turned-scientific-paper was on leaf anatomy of Salvia apiana, Salvia mellifera, and their hybrids [ PDF ]. The introgressive hybridization of these two species has become something of a legend—the species have different chromosome numbers, but nevertheless can form backcrosses. These two quite distinctive species and hybrids between them were readily available in Claremont, so it was an easy choice for a research topic. The leaves of the two species are quite different in appearance and texture, so not surprisingly, the anatomy of the leaves differs also. One can detect proportionate parentage of a given hybrid plant by means of leaf anatomy alone.
Synthesis versus specialization. Leaf anatomy continues to be an interesting subject that tends to be neglected. Leaf anatomy is such a sensitive indicator of physiology and ecology of a plant that one is amazed that studies this field are not more common. The answer may actually lie in the tendency to separate plant anatomy and plant physiology. Botanists know that the two fields are closely related, but all too often, students of plants are not trained enough in both physiology and anatomy for those students to feel at ease using research tools in both of those fields. Botanists tend to gravitate toward the research tools and research areas in which they feel most competent, thereby narrowing their approaches. Many of the advances in evolutionary biology (and science in general), however, are made by syntheses between fields that tend to become separated but which are relevant to each other. If a paper representing a synthesis is written by a single author but reviewed by two different people expert in each of two fields involved, it will probably be criticized for not having been written according to the reviewer’s specialty! Singly-authored papers are becoming less frequent. There’s nothing wrong with collaboration, but if synthesis is involved, one of the authors needs to work at creating that synthesis.