How Plants Resist Hostile Forces Around Them 271 



spiration is much less from vertical surfaces, since the force of 

 the sun which supplies the transpiration energy is obviously 

 much less powerful upon vertical than horizontal surfaces. 

 Doubtless, by the way, this factor is much more potent than 

 those of protection against light and heat in determining the 

 prevailingly vertical position of green tissues, whether of stems 

 or of leaves, in plants of dry and desert places. And this con- 

 clusion is strongly confirmed by the fact that salt marsh plants, 

 which need protection against much transpiration though hardly 

 at all against light and heat, especially in northern regions, show 

 a notable tendency to a vertical position of leaves and other green 

 tissues. But the very same end is also attained in a different 

 way by the provision, outside of the stomata, of chambers in 

 which the escaping vapor is held for a time, thus checking tran- 

 spiration a little, somewhat as a damp atmosphere would do, 

 while the inward diffusion of carbon dioxide is not appreciably 

 affected. In some plants these chambers consist of deep pits in 

 the thick epidermis with guard cells lying at the bottom; in 

 others the same effect is produced by coatings of hairs or scales; 

 while in still others the leaves are inrolled to tubes into which the 

 stomata all open. The same result follows, as well, from the 

 dense crowding together of leaves, such as desert plants show not 

 infrequently (figure 12, center). And many other arrangements, 

 notably hardness of tissues, and the presence of gelatinous sub- 

 stances, both contributing to water conservation, have been 

 described. 



Thus much for dangers from dryness; the other extreme, — 

 too much water, — likewise constitutes at tunes a danger to 

 plants, rarely, however, in any direct manner, but indirectly 

 through prevention of the access of air supply. But this matter 

 has already been considered along with respiration and aeration, 

 where the various protective adaptations (air passages, aerating 

 structures, utilization of the dissolved air of the water), — have 

 been sufficiently described. A very different kind of protective 



