270 



The Living Plant 



amount proportional to the food manufactured. This inevitable 

 linking of transpiration with photosynthesis is one of the most 

 fundamental of all facts in the economy of green plants. Some 

 plants of dry places, especially the deserts, have solved the 

 adaptive problem thus presented by condensing all of their 

 photosynthetic work into the brief moist season (for all deserts 

 where plants can grow at all do possess such a season), spending 

 the remainder of the year in a resting and dried state, com- 

 parable with that assumed by our plants over winter. But 

 others, illustrated by the Cactuses conspicuously, continue their 

 activity all through the season, in reliance upon a copious store 

 of water and sundry devices for rigid economy of the same. 



This matter of economy in transpiration is so important and 

 interesting that we must give it a little further consideration. 

 The first and most obvious method thereof consists in the re- 

 duction of total green surface, which of course is carried to a 

 degree sufficient to keep the unavoidable loss within the hmit of 

 safety. This is the reason why the plants of dry places, and 

 especially of the deserts, are comparatively small, why they are 

 so commonly compacted in form, and why they so often are 

 leafless, — the very object of the existence of the leaf, as the 

 reader will recall, being that of spreading more surface. A 

 second method consists in the provision of an especially efficient 

 epidermis, preventive of transpiration except through the regula- 

 ble stomata; and so thick and strong does it become in some 

 plants that it is hard to cut and impossible to compress with the 

 hands, and actually resembles a coating of horn spread all over 

 the plant, as some of the Cactuses illustrate. But a third and 

 most interesting method of transpiration economy consists in 

 certain arrangements which hinder somewhat the transpiration 

 without interfering with the gas diffusion. This to some degree 

 is accomplished by a vertical position of the green tissues, for 

 while the diffusion of carbon dioxide through the stomata takes 

 place with equal facility in any position of the tissues, the tran- 



