238 



PHYSIOLOGY OF GROWTH AND CONFIGUEATION 



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

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



,^_^.^__^ Fig. I20, D, is shown a cross-section through the 



^ ^r]r\r\r]r\r ^su. epidermis of a leaf of Lupinus mutdbilis from a culture 

 lir''-^L>'UL/Lj in dry air, a corresponding section of a leaf grown 

 jQQuOC ^'^ raoht 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 dandeUon (Taraxacum 

 ^\(^r^y~|/^(^"'Jj officinale) grown in a nearly saturated atmosphere is 

 M:^=^^^^^&-i shown in Fig. 121, A, similar ones grown under usual 



, B and B' 

 often 



conditions being shown in Fig. 121, 

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

 epidermitioi Lupinus mutabiiis. thorns, and if such plants are grown in a verv moist 



D, grown in dry; M, in moist , , ^i ,-i „ j ^>. 



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. 



Wiesneri has demonstrated that there may be a descending as well as an 

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

 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 transpiring 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 pecuharities 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 



> Wiesner, J., Der absteigende Wasserstrom und dessen physiologische Bedeutung. 

 -D, 24-29. 1889. 



Bot. Zeitg. 47: 



