ABSTRACT
The first text is the book’s introduction, which is divided into four chapters, on “Floral and ecological plant geography“, “Forms of life (forms of vegetation)“, “Plant societies” and “Survey of the contents of the following chapters“. Floristic plant geography has to enlist species in greater or smaller areas and then to subdivide these areas into smaller units or zones according to a variety of biological, geological and geographical conditions. Ecological plant geography teaches how plants and plant societies settle their form and economic conditions according to the factors acting upon them from outside (such as heat, light, food, water).
The concept of “growth form” designates “the state of the adapted plant“, that is, to use the formulation of Julien Vesque, its “epharmony“. This is a physiognomic concept. It corresponds to that of the “vegetation form” Which originates with Grisebach, but it was Alexander von Humboldt who was the first to grasp the significance of the relationship that exists between the physiognomy of the vegetation and the landscape. Warming thus presents several typologies of growth forms, including that of Humboldt, who distinguished nineteen, and Grisebach, who developed sixty. The importance of the concept of growth form for an ecologically oriented geobotany is emphasized once again.
Subsequently, geobotany had to turn to the consideration of natural plant communities; these include numerous species with quite variegated growth forms.
The very detailed outline of the work includes the following pans: ecological factors and their impact; cohabitation and plant associations; hydrophyte associations; xerophyte associations; halophyte associations; mesophyte associations; and, finally, in pan seven, competition between the plant associations. This last pan concludes with a discussion of the origin of species.
828 The second text is a chapter on the application of ecological plant geography: an extended description of a special form of vegetation exposed to soils rich in salt, i.e., the plant societies of “halophytes“. Wherever the soil is rich in salt, a very special vegetation appears which is made up of only a few distinct families with a peculiar morphology and anatomy. Nonetheless, there is an extraordinary concordance between the halophytes and the xerophytes: succulence, presence of mucilage cells, and reduction of the plant surface - all of which have the effect of reducing transpiration. Schimper attempted to explain this phenomenon: if transpiration is reduced, rises in salt - which is toxic in very high concentrations - are also diminished. Warming contests this explanation: it seems more likely that plants that are capable of tolerating a high salt content in their cells become halophytes. In addition, Stahl indicated that the stomata can close entirely in non-ha-lophytes; however, this is not the case of stomata in halophytes, which remain open even when the plants wither. The author therefore concludes that the halophytes must be separated from the xerophytes. (...)
The third text is entitled, “The Origin of Species“. Plant species change in accordance with changes in their environments. It had been proposed that these transformations might be at the origin of new species. The first condition for a new species to be able to develop from another is its plasticity. Some believe that the mechanisms of natural selection are also operative. In line with the previous discussions in the book, the author considers that plants have the capacity to adapt directly to new conditions of life. The question of the direct influence of the conditions of life on biological forms is taken up, including, in particular, several examples of the variability of biological forms in accordance with light and humidity. The question of plant transpiration is examined. Numerous examples of direct adaptation to various external factors, such as available food, are given.
Not only can plants vary morphologically, but they also vary in terms of their biological particularities. Frost can weaken a plant, and climatic variations can alter vegetative rhythms (foliage, dormancy). Plant metabolism is undoubtedly subject everywhere to the laws of adaptation and self-regulation. Naturally, all plants do not have the same plasticity.
Direct adaptation is undoubtedly a very important factor in speciation. But it is not the only one. There also exists what Julien Vesque called “phyletic variability“, which is a hereditary variability of the species that is not related to the environment. A third factor is natural selection, as expounded in the Darwinian hypothesis. A fourth factor is the crossing of different varieties. The scope of activity of direct adaptation seems to be focused on plant organs and metabolism. Direct adaptation is a result of adaptations that have taken place over generations while, simultaneously, heredity consolidates characteristics acquired of greater or lesser importance. From this point of view, Lamarck showed more insight than the others. Direct adaptation is one of the most powerful evolutionary factors in the organic world.
