20 



THE FACTORS 



[Part I 



As a means for promoting transpiration the significance of a well- 

 developed air-containing intercellular system, such as exists in the leaves 

 of all hygrophytes (Figs. 25, 26), is self-evident. The very numerous 



FlGS. 25 and 26. Hygrophilous structure. Leaf of Fagus sylvatica. Left hand : Transverse section 

 of a shaded leaf. Right hand : Spongy parenchyma of a shaded leaf. After Stahl. 



stomata serve as efferent passages, and they are not deeply sunk in the 

 surface, as in xerophytes, but are superficial in position, and sometimes 

 even raised on cones without any protective arrangements. 



HydatJiodes (Fig. 27), which are organs for excreting liquid water, whose 

 wide distribution, variety, and great importance were first revealed by 



Haberlandt 1 , are specially 



characteristic of hygro- 

 phytes in a very damp cli- 

 mate. Early in the morn- 

 ing under such climatic 

 conditions, especially in 

 the tropics, many plants, 

 herbs as well as trees, are 

 so covered with drops of 

 water that not infrequently 

 a drizzling rain seems to 

 be descending from the 

 forest canopy of leaves. 

 This has been incorrectly 

 regarded as dew. It owes 

 its origin much more to 

 excreting hydathodes, the 

 activity of which is greatly 

 increased when transpira- 

 tion is restricted, but ceases when the air is dry. Hydathodes are epi- 

 dermal structures of the most varied nature. Sometimes simple, sometimes 

 complex in structure, they appear in the form of hairs, glandular cells, 



1 Haberlandt, II and III. 



Fig. 27. Hygrophilous structure. Hydathodes of leaves. 

 a and b Gonocaryum pyriforme. c Peperomia exigua. Mag- 

 nified. After Haberlandt. 



