36 THE FACTORS [Part I 



of the sclerotic elements, which however shows itself only in still water and 

 in still air. 



The great plasticity which rendered possible the change of terrestrial 

 plants into aquatic plants has to some extent been inherited by descendants 

 from their ancestors. Most phanerogamous and fern-like aquatic plants can 

 still change into terrestrial forms approaching ordinary terrestrial plants in 

 structure (Figs. 30-32). Shorter axes, less subdivided leaves, palisade- 

 parenchyma, stomata, a well-developed cuticle, and centripetal disposition 

 of the chlorenchyma distinguish such terrestrial forms from aquatic forms. 

 But many of the characteristics acquired during a normal life in water have 

 become hereditarj', such as the central position of the vascular bundles. 

 In fact the whole structure show^s unmistakably that we have to deal with 

 somewhat modified aquatic plants. Such deserters from the water generally 

 remain weakly, and rarely or never blossom, as opposed to true amphibious 

 plants in which it is precisely the terrestrial form that displays sexual 

 activity more frequently than does the aquatic form. 



Terrestrial plants that have been transformed into aquatic plants com- 

 prise only a small section of the whole aquatic flora. Algae, which we 

 have not yet considered, preponderate in aquatic vegetation, and are true 

 aquatic plants whose ancestral forms have always inhabited water. Even 

 the few existing terrestrial Algae are more dependent on liquid water than 

 are true terrestrial plants. These qualities render Algae less suitable than 

 more highly organized types for illustrating the difference between aquatic 

 and terrestrial plants. 



3. WATER AND REPRODUCTION 1. 



A plentiful water-supply, as a rule, favours the development of the 

 vegetative organs ; scarcity of water brings about their reduction. On the 

 contrary, ilie prodjiciioii of sexual organs is usually impeded by a considerable 

 supply of moisture and favoured by drought. 



This principle, which has long been established by practical operations, 

 has led to various horticultural artifices for the production of a rich supply 

 of blossom. Among these, for instance, is the art of root-pruning, in which] 

 a trench is dug around the plant and the exposed portion of the root- 

 system cut off. In Ceylon, in order to cause the vine to blossom, the roots; 

 are for a time partially laid bare. Cereus and other Cactaceae bear flowerss 

 more plentifully if they have been shrivelled up for some time, than after| 

 a period of uninterrupted turgescence. Many plants, for instance certain 

 species of Juncus, blossom only in a relatively dry soil. 



Retarded passage of water through the vessels leads to similar results 

 If a twig of a coffee-plant be broken so that it remains attached to a branch 



' Kumerous references in Sorauer, Pflanzenkrankheiten, Vol. I, and Moebius, op. cit. 



