PHYSIOLOGICAL AND ECOLOGICAL ADAPTATIONS 7 



differentiation of a palisade-shaped photosynthetic element, and the 

 special modes of construction employed for this purpose in different 

 plants are adaptive only in so far as they tend to this common end. 

 Similarly, most of the various (sectional) plans of arrangement embodie< I 

 in the mechanical systems of inflexible organs are merely expressions 

 of this tendency towards variation of design, and must, as far as 

 their details are concerned, be treated as characters of purely morpho- 

 logical value. 



All the phenomena with which the physiological anatomist is con- 

 cerned partake of the nature of useful adaptations. They may be 

 subdivided into two groups, which are, however, by no means sharply 

 delimited from one another. In cases of physiological adaptation the 

 morphological feature under consideration whether it be a localised 

 structure or an entire tissue-system is intended to perform a physio- 

 logical function that is to play a definite part in the internal economy 

 of the plant. But there also exist numerous ecological (or biological) 

 adaptations, which are developed in connection with the manifold 

 requirements arising out of relations between the plant on the one 

 hand, and environmental factors, such as climate, habitat, and the 

 animal world, on the other. 



Usually it is the physiological adaptations that mainly determine 

 the structure of the several organs and tissue-systems, ecological 

 adaptations being of minor importance in this respect. The photo- 

 synthetic system, for example, displays specialisation principally in 

 connection with the physiological process of photosynthesis, as indeed 

 its name implies. Its structure is such as to provide for the accom- 

 modation of the largest possible number of chloroplasts, and for the 

 rapid removal of synthetic products.. At the same time a certain 

 amount of adaptation to external conditions, such as the intensity of 

 illumination and the humidity of the atmosphere, is also discernible 

 in the construction of the photosynthetic system. Another illustration 

 is afforded by certain characteristics of the vessels that serve for the 

 conduction of water. Here the presence of bordered pits, and, in 

 fact, the peculiar mode of thickening of the cell-wall in general, is 

 closely connected with a particular physiological process, namely, the 

 ascent of sap. The number and the diameter of the vessels, on the 

 other hand, are dependent upon ecological requirements, which in their 

 turn may be determined by the nature of the habitat if, for example, 

 one compares a land plant with an aquatic species or by some 

 peculiarity of habit, as in the case of climbers. More rarely the 

 appearance of a morphological feature is correlated mainly, or even 

 entirely, with some ecological requirement ; this condition is exemplified 

 by the dermal system, the principal function of which is protection. 



