﻿The tissues underneath such regions are usu- 

 ally free from chlorophyll. The leaf might be 

 compared to a sheet of tinted glass which per- 

 mits the passage of the greater part of the light 

 which falls upon it while intact. When 

 crushed into fragments the mass of minute 

 fragments reflects back the rays at thousands 

 of points. 



Plants such as the aloe, growing in the 

 fierce light of a tropical desert, avoid burning 

 and dr3ang of the tissues by such arrange- 

 ments. The silvery areas w^arm up much 

 more slowly than a typical leaf. At the same 

 time this feature is also of use to plants grow- 

 ing in moist, tropical climates, in the promo- 

 tion of transpiration at night. 



The silvery areas not only warm up slowly 

 but they also radiate heat less rapidly than a 

 typical leaf, and the higher temperature of 

 these areas after sunset promotes transpira- 



3. Transverse section of a velvety leaf of Piper porphyr- 

 acexim. A layer of aqueous tissue lies next the epidermis of the 

 upper and lower sides. The anthocvan is in the lower half of 

 the leaf. 



■!■. Mottled leaf of Begonia falcata. a. Transverse seetion 

 of a brownish green velvety shining portion of lamina. The 

 epidermal cells of the upper side are furnished with papillose 

 extensions. The epidermal and sub-epidermal layers are joined 

 without intercellular spaces. The epidermis of the lower side 

 and the spongy parenchyma contain anthocyan. b. Trans- 

 verse section through a silvery portion. The outer walls of 

 the epidermis are plane. Large air spaces are present between 

 the epidermis and the cells containing chlorophyll. 



^. Transverse section of a bright spot on the leaf of Ranun- 

 culus ficarioides. The sub-epidermal cells, a, contain a few 

 small chloroplasts, and are seiDarated from the layer beneath 

 by large air-spaces. 



6. Papillose epidermal cells of Begonia imperialis, var. 

 smaragdina, seen from above by refracted light. After Stahl. 



Silvery areas 



