How Plants Resist Hostile Forces Around Them 267 



but cool days of early spring, when all the warmth procurable by 

 the plant is desirable for hastening the development of the 

 various parts. This explanation has been applied in particular 

 to the red stigmas and styles of wind-pollinated flowers which 

 ripen before the appearance of the leaves in the spring, the extra 

 warmth thus acquired being supposed to promote the growth of 

 the pollen-tube and hence to hasten the fertilization. But here 

 we are nearing mere guesswork, and must not accept such sug- 

 gestions as explanation, but simply as interesting hypotheses 

 deserving of determination through the test of experiment. 



We come now to the most deadly of all the dangers to which 

 plants are exposed, and that is dryness As the reader will 

 readily recall, water is not only the principal constituent of the 

 bodily structure of plants and indispensable in their daily nutri- 

 tion, but is also evaporating or transpiring, constantly, co- 

 piously, and unavoidably, from all of their younger aerial parts. 

 Therefore plants need a constant and uniform water supply, but 

 in fact rarely get it, for the most of the kinds, including all of 

 those most familiar to us, live under conditions of extreme 

 variability not only as to the quantity available for absorption 

 by the roots, but also, and especially, as to the quantity forcibly 

 transpired from their tissues, these conditions, indeed, being 

 linked with the most variable of all things, the weather. Against 

 such fluctuations ordinary plants secure a tolerable protection, 

 on the one hand through their power of absorbing even the 

 hygroscopic water of the soil through their copious root hairs, 

 and, on the other, by their complete waterproof epidermis, the 

 few necessary openings in which are automatically regulated, 

 albeit somewhat clumsily, in adjustment to the prevailing condi- 

 tions. But in places where water is permanently scant, as it is 

 extremely in deserts, these simple arrangements are insufficient 

 and must be supplemented by special protective adaptations; 

 and these take three different forms, under which heads we shall 

 consider them, (a), increased efficiency of the absorbing system, 



