How Plants Resist Hostile Forces Around Them 263 



rapher needs a total exclusion of blue rays and therefore a screen 

 of much deeper color than the plant requires for only a partial 

 exclusion of those rays. Such is most likely the adaptive signifi- 

 cance of that charming red blush which mantles the face of the 

 fresh vegetation of spring, for, without some such protection, 

 the young leaves and stems that push out of the buds before 

 the formation of the chlorophyll, which constitutes later a suffi- 

 cient though incidental protection, would expose then- unshielded 

 protoplas/n to the full force of the bright light then prevailing. 

 And there are some students who find a similar function in the 

 redness of leaves in the autumn, believing that it shields the 

 protoplasm after the chlorophyll has faded away; though here, as 

 I believe and have argued in the second chapter, there is little 

 warrant in the evidence. Certain it is that there are cases, e. g., 

 the red under sides of leaves of some tropical undergrowth 

 plants, where the explanation must be totally different. But 

 the light-screen function explains very well the reddish or brown- 

 ish colors of spores which must float long-time in the air exposed 

 to the brightest of light, and perhaps it explains also the red 

 color assumed by roots and underground stems when these be- 

 come exposed to the light, though here the color may represent 

 simply a chemical incident. 



A second method of light protection may consist in those hairy 

 or woolly coatings, or even in the waxy or resinous layers, which 

 overspread a good many plants of open bright places, resulting 

 in a distinctive aspect of grayness found especially often in plants 

 of the deserts. Such covers must act to reflect and refract the 

 light, without, of course, any distinction of rays, to an extent suf- 

 ficient to weaken very greatly its power to penetrate the tissues. 



The third of the methods of light protection, bound up, how- 

 ever, with protection against excessive transpiration soon to be 

 noted, is more important. It consists in the assumption by the 

 green tissues of a vertical position, whereby they present only a 

 thin edge, or at least a low angle of incidence, to the mid-day 



