IxNFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 233 



On the other hand, some gases have a stimulating effect upon growth. In 

 Johannsen's^ experiments, bulbs sprouted much more rapidly in an atmosphere 

 containing ethyl ether than in one lacking it. Johannsen recommended treat- 

 ment with ether as a method for forcing plants. In Fig. 112, A, is shown a 

 branch of Syringa (Ulac) in November, eight days after treatment with ether 

 vapor; the fifth twig from the left was protected from contact with the gas. 

 In Fig. 112, B, the same branch is shown three weeks after treatment, and is in 

 full bloom excepting that the twig originally, untreated (here also on the right) 

 still remains leafless. 



§4. Influence of Moisture on Growth and Configui;ation. — The condition of 

 the soil and that of the air, with respect to water, determine the amount of 

 water absorbed and also its rate of movement 'through the plant. When the 

 atmosphere is saturated with water vapor, transpiration from the leaves is 

 materially lessened and, consequently, the further absorption of water by the 

 roots is similarly decreased. Dry air, on the 

 other hand, accelerates both transpiration 

 and water absorption. 



Plants grow luxuriantly only with a 

 plentiful supply of water. Tropical vegeta- 

 tion is exceptionally luxuriant since an 

 abundance of water is here combined with 

 favorable temperature conditions. The 

 virgin forests of the tropics are frequently 

 impenetrable jungles, where plants grow not 

 only on the soil but even on each other 

 (epiphytes). It is quite different with arid 

 regions; the plant world here maintains only ,. ^ „ t 



o ' r- 1 • 1 1 J Fif'- 113- — A areinca 01 Rubus squarrosa 



a scanty existence. Parallel with the de- (i^ natural size). {After Wiesner.) 

 creased number of plants occurring in arid 



regions, the individual plant also is smaller than in humid regions. Plants of 

 moist regions have well-developed foUage, often with very large leaves that have 

 a high water content. Plants in dry climates have relatively small leaf surfaces, 

 so that the loss of water is not so great. Thus the leaves of Rubus squarrosa 

 (Fig. 113), which is closely related to the European raspberry {Rubus idaus) 

 are very small. Many xerophytes, such as the cacti., have no leaves at all, or 

 they lose them very soon after they are formed. In this case the activities 

 that are usual for leaves occur in the stem. Such plants are furnished with 

 many arrangements that hinder the loss of water. The epidermis is very tough, 

 frequently possesses hairs, and is often covered by wax and other incrustations. 

 Thus Rochea Jalcata, a South African plant, is armed with a siliceous coat of 

 mail. A cross-section of the leaf shows that the small cells of the epidermis 

 are overlaid with a continuous layer of large, bladder-like cells (Fig. 114), the 

 walls of which are richly impregnated with silica. These siliceous cells are filled 

 with water, which is replaced by air only when they become old. As long as 



' Johannsen, Wilhelm L., Das Aetherverfahren beim Fruhtreiben. 2te Aufl. Jena, 1906. 



