48 A TEXTBOOK OF BOTANY [Ch. Ill, 6 



lowed by a revival of the leaves. It is apparently a similar 

 excess of transpiration over absorption or conduction which, 

 no matter how abundant the root water, limits the kinds 

 of plants we can grow in the dry air of our houses; for 

 house plants, as well known, are not so much those we want 

 as those we can make grow. It is clearly the defective 

 absorption by roots, which absorb slowly at low tempera- 

 tures, in conjunction with excessive transpiration, which, on 

 bright, dry, windy days in early spring, causes the drying, 

 browning, and death in ornamental evergreens ; and likewise 

 a wilting, browning (called wind-burn), and death, in the bud- 

 ding foliage of deciduous plants. The winter-killing of 

 shrubs, as we shall see later, is also largely identical in nature. 

 But the effect of light, heat, dryness, and winds upon tran- 

 spiration shows most clearly of all in the vegetation of those 

 parts of the earth where such conditions prevail in conspicu- 

 ous intensity, — the deserts. For there, as well known, and 

 represented in pictures in Part II of this book, the thin- 

 leaved, open types of plants cannot grow at all, and only 

 those sorts can manage to exist which are compact and 

 thick of texture, or have other transpiration-limiting fea- 

 tures. The aggregate effect is the peculiar and even some- 

 what bizarre appearance characteristic of desert vegetation. 

 What now is the physiological meaning of transpiration, 

 this water-loss which cannot be wholly stopped even though 

 at times it endangers the existence of plants, and greatly 

 restricts their distribution? The cellular anatomy and 

 physiology of leaves give the answer. All chlorenchyma 

 tissues are continually saturated with water, the direct evap- 

 oration of which is prevented by the waterproof epidermis. 

 This epidermis is practically impermeable to the carbon 

 jdioxide required by the leaves in their food-forming function, 

 and also to the oxygen released in that process ; but the access 

 and exit of those gases take place through the stomatal 

 openings. When these stomata are open for such gas 

 passage, however, there is nothing to prevent the water of 



