2 68 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



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The leaves formed by Tropaeolum plants growing in moist air and moist 

 soil were thus five times as large as those formed in the driest cultures. In 



Fig. 120, D, is shown a cross-section through the 

 epidermis of a leaf of Lupinus mutabilis from a culture 

 in dry air, a corresponding section of a leaf grown 

 in moist air, being shown in Fig. 120, M. The dif- 

 ferences in the thickness of cell wall and of cuticle 

 are very great. A leaf of the dandelion (Taraxacum 

 officinale) grown in a nearly saturated atmosphere is 

 shown in Fig. 1 2 1 , A , similar ones grown under usual 

 conditions being shown in Fig. 121, B and B' . 

 Fig. 120.— Sections of leaf Plants growing in dry regions often possess 

 epidermis of Lu pinn. s mniabiiis thorns, and if such plants are grown in a very moist 



D, grown in drv; M, in moist . , ,-, ,, ,, , , , 



air< atmosphere the thorns are generally replaced by 



short, leafy branches. Two branches of broom 

 (Genista anglica) are shown in Fig. 122, one (C) grown under normal conditions, 

 the other (B) grown in moist air. The difference is so great that they might 

 be taken to be distinct species. 



Wiesner 1 has demonstrated that there may be a descending as well as an 

 ascending water stream in plants. The presence of the former may be clear- 

 ly demonstrated in the following way. A cut branch of grapevine or similar 

 leafy shoot is placed with the youngest, terminal portion of the stem in water, 

 while the rest projects into the air. After some time the part of the stem 

 under water wilts, which is explained by the fact that the actively trans- 

 piring leaves remove more water from the terminal portion than it can absorb, 

 in spite of the fact that it is surrounded by water. 



Many structural peculiarities of plants may be explained as due to the 

 descending water current. For instance, in many plants a withering of the 

 'terminal bud occurs, with the consequent formation of a sympodium. The 



1 Wiesner, J., Der absteigende Wasserstrom und dessen physiologische Bedeutung. Bot. Zeitg. 47 : 

 1-9, 24-29. 1889. 



