346 EXPElilMENTS WITH PLANTS 



217). The leaves of the compass plants are vertical 

 and directed north and south. Many leaves tempora- 

 rily assume a vertical position in very strong sunlight 

 (see page 217), and it is noticeable that young leaves, 

 which are more sensitive to the light than the older 

 ones, usually assume this position. (Plants can be 

 grown by electric light ; and this has proved profitable 

 in forcing certain greenhouse crops in winter.) 



The trunks of trees may suffer from "sunscald," due 

 to injury of the bark by the intense light and heat of 

 the sun, or to the alternate thawing and freezing of the 

 bark on the southwest side in late winter. The trouble 

 may be obviated by shading the trunk with a screen of 

 wire, boards, cornstalks, etc. Pruning often lets in too 

 much light on the trunk, which suffers in consequence 

 (this is especially noticeable in the Weeping Willow) . 



Very interesting results are obtained by growing- 

 plants in darkness. To do this out-of-doors, it is only 

 necessary to cover the plant with a section of stove- 

 pipe, a wooden box or a barrel. Within doors the 

 plant may be placed in a dark closet or box. Such 

 plants (Figs. 103 and 180) have very long, slender 

 stems (the nodes are far apart) ; they are pale in 

 color, have little woody fiber, ^ and are consequently 



1 This fact is taken advantage of in bleaching Asparagus, Celery and 

 Rhubarb for market. The Celery is either "hilled up " or covered with gunny- 

 sacks, or the light is excluded by means of boards : the leaf-stalks are thus 

 bleached and rendered tender and succulent. The "laying," or lodging of 

 Wheat is due to the shading of the lower portion of the stalk, which results in 

 an insufficient production of woody fiber. 



