How Plants Resist Hostile Forces Around Them 265 



extreme degree represented by the Smilax. This very over- 

 turning does actually occur in the Cactus family, in which, 

 happily, all of the steps without exception are represented by still 

 living forms. It is the relics, indeed, of such devious windings in 

 the past history of plants which give us our principal morpho- 

 logical puzzles. 



This consideration of light naturally suggests the question as 

 to heat. This, likewise, is indispensable to plants, since it supplies 

 a condition requisite for some chemical reactions and physical 

 movements, notably diffusion. Heat also, like light, is more 

 and more useful up to a certain intensity (about that of blood 

 heat in ourselves), beyond which any increase is not only without 

 benefit, but soon becomes an injury. Thus, plants in the fields in 

 summer by no means thrive better the hotter it gets. It is doubt- 

 ful, however, whether the natural heat of the sun ever attains 

 an intensity dangerous to plants, and even if it does, the same 

 structural adaptations, especially refractive coverings and a 

 vertical position of green tissues, protective against light, would 

 be equally effective against heat. And there is perhaps yet 

 another method of protection against both, but especially heat, 

 namely transpiration, which dissipates through evaporation the 

 too intense energy of heat and light thrown into the leaf by the 

 sunlight, as we have noted already (page 209). There is, how- 

 ever, one place on the earth's surface, and that is in hot springs, 

 where low kinds of plants belonging to the Algae can grow at a 

 much higher temperature than the sun ever produces, even a 

 degree too hot for the hand to endure (up to 81 Centigrade or 

 192 Fahrenheit). In these Alga? no structural adaptations to 

 protection occur (unless a certain slimy coating be such, though 

 this is hard to believe), but the living protoplasm has apparently 

 become acclimatized to the high temperature, probably by the 

 elimination of all chemical constituents affected thereby and the 

 substitution of others whose reactions are under full control at 

 such temperatures. 



