478 MANUAL OF BOTANY 



upon the growing organ. The inherent tendency just spoken 

 of can be satisfactorily seen only when, by artificially' eliminat- 

 ing the action of such forces, the plant is not exposed to their 

 stimulating influence. Such a tendency has been called 

 Rectiijetality. 



CHAPTEE XIII. 



INFLUENCE OF THE ENVIRONMENT ON PLANTS. 



According to the nature of their surroundings and the con- 

 sequent differences in mode of life, we find in many plants 

 certain peculiarities of form and structure which are different 

 from those of the bulk of the forms which we have hitherto con- 

 sidered. Of these the Spermaphytes which live in water may 

 be first discussed, as the direct influence of their environment is 

 most conspicuous in their case. 



These aquatic Phanerogams may be divided into two chief 

 groups ; those which are altogether submerged, and those which 

 bear floating leaves as well as or instead of submerged ones. 



In the former case the plant body may be attached bj' roots 

 to the bottom of the stream or pool, or maj^ be altogether 

 floating. The stems are almost invariably' long and slender, 

 and easily swayed to and fro in the water. They depend for 

 their support upon the nature of the medium in which they 

 live, and though possessing a certain rigidity-, this is not asso- 

 ciated with any great developmeni of woody tissue. Generally 

 the latter is reduced to a minimum ; the fibro- vascular bundles are 

 usually few and contain few lignified elements. The substance 

 of the plant is largely parenchj-matous, and the cells have thin 

 walls. The intercellular space system is often very complex, 

 large lacunae filled with air occupying a large space in the 

 distribution of the tissues. Their rigidity is secured by the 

 turgescence of the parenchymatous cells, and buoyancy is much 

 assisted by the air in the lacunae. 



The primary root is generally feebly developed, and, as a 

 rule, does not persist through the life of the plant. Adventitious 

 roots, however, are given off in large numbers from the various 

 nodes of the stem. The root-hairs which are so characteristic 

 of terrestrial roots are usually either very scanty or altogether 

 absent. The epidermis of both root and stem is not cuticularised. 



