100 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



shape and anatomy. This, according to MacCulland, is directly due to 

 the arrest of transpiration and to the cells being overcharged with water. 

 12. Duration of life. The vast majority of water-plants, at least 

 among spermophyta, are perennial ; this is in accordance with the favour- 

 able environment, which is but slightly affected by seasonal change. 

 Exceptions are, however, provided by many Cryptogamia and some 

 vascular plants, such as Salvinia, Naias, and Subularia. Vegetative 

 propagation far exceeds sexual reproduction in many water-plants : 

 this may go to such a length that production of fruit is entirely elimin- 

 ated. Certain species, such as Elodea canadensis (at least in Europe, 

 where only the female plant occurs), many species of Lemna, and others, 

 multiply exclusively in a vegetative way. It is a general biological 

 phenomenon that humidity opposes the production of sexual organs, 

 whereas aridity promotes it. 



The peculiarities of water-plants that have been mentioned here 

 are to be interpreted in general as examples of degeneration, and of 

 morphological and anatomical retrogression, if we compare water-plants 

 with land-plants ; this retrogression we may, with Henslow, regard as 

 adaptive.* 



CHAPTER XXIX. ADAPTATIONS OF LAND-PLANTS 



The land-plant contrasts most strongly with the water-plant as 

 regards external and internal construction, in that parts in contact 

 with air the assimilatory organs in particular are exposed to transpira- 

 tion, and must therefore be adapted in an entirely distinct manner. 



Transpiration is a physiological process determined by factors of 

 two kinds : external or environmental, and internal or those dependent 

 on the precise structure or temporary condition of the plant. 



The external (climatic and edaphic) factors were mentioned in Section I : 

 they are more particularly isolation, temperature, saturation-deficit, and 

 movements of the atmosphere. The supply of water depends upon 

 the nature of the soil, including quantity of water, temperature, acidity, 

 amount and concentration of the salts in the soil. Correctly speaking, 

 as Clements ^ insists, climatic factors to a great extent affect conditions 

 in the soil. 



As regards internal factors, transpiration depends on the dimensions 

 of the evaporating surface, and inasmuch as it is the foliage-leaves through 

 which evaporation mainly takes place, it is likewise the size, disposition, 

 and thickness of the foliage-leaves, as well as the whole development of 

 the aerial shoot, that above all determine the amount of transpiration ; 

 this is also influenced by the nature of the epidermis (cuticle, wax, cork, 

 hairs, and stomata). The foliaged shoot gives the clearest indication of 

 the conditions under which the plant has developed. An additional 

 determinant factor is the nature of the root-system ; the larger the absorb- 

 ing surface is, the more water can there be absorbed in the same time ; 

 and the deeper the penetration of the root, the greater is the certainty 

 that the supply of water will not be cut off by drought. 



The regulation of the amount of water within the plant is accomplished 



* Henslow, 1895. ^ Clements, 1904. 



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