EFFECTS OF LIGHT INTENSITY 729 



organs were progressively more attenuated than those from normal 

 organs. The length of time which plants may be kept growing in dark- 

 ness depends upon the amount of stored food available and also on the 

 temperature at which they are grown. At high temperatures the plants 

 exhaust the food supply more rapidly. MacDougal kept plants grown 

 from root-storage organs alive for as long as 18 to 20 months. Plants 

 grown from seed, on the other hand, generally succumbed in much 

 shorter periods, owing to lack of stored food; however, a seedling of the 

 cocoanut palm lived 15 months in darkness and still had exhausted only 

 half the endosperm. 



Leaves of monocotyledons with parallel veins tend to elongate in 

 darkness at the expense of growth in width. They are frequently rolled 

 or folded. Plants with open or netlike venation usually develop exces- 

 sively long petioles in the dark, with small narrow leaves, often more or 

 less folded (65, 77, 89). Climbing plants and tendrils have a tendency 

 to elongate excessively in darkness and seldom develop the typical cork- 

 screw growth of plants grown in the light. If the tendrils are placed in 

 contact with a support, they will make one or two turns around it but 

 then continue to elongate (65). 



Plants grown from buds, corms, or other organs in which the leaf, 

 stem, and flower primordia are laid down the previous season, tend to 

 produce all these organs in darkness the first season of etiolation. Plants 

 grown from seed, or storage organs formed while the plant was kept in 

 the dark, do not develop flowers, nor do they usually attain the same stage 

 in development attained by normal plants during the same period (65). 



Some unusual structural features were noted by Brown (16) in 

 etiolated shoots of Opuntia Blakeana, a prickly pear cactus. The etio- 

 lated stems were rounded or oval in cross section, as noted by MacDougal 

 (65). The epidermal cells were elongated, especially near the apex of 

 the joints. The cuticle was very thin, or absent, the cortical layer weakly 

 developed, and the palisade tissue imperfectly organized. Under the 

 epidermis, in normal plants, there is a row of cells containing crystals 

 of calcium oxalate. These crystals were absent in etiolated shoots. Of 

 particular interest were the modifications of the stomata. In normal 

 plants these are sunken below the surface, while in etiolated shoots he 

 observed that they were raised above the surface on minute papillae 

 composed of from 6 to 12 cells or more. These papillae w^ere apparently 

 formed from the stomatal mother cell which continued division, instead 

 of dividing only once, to form the guard cells. The guard cells were 

 elongated and wedge-shaped in the etiolated shoots. Some well devel- 

 oped stomata were found beneath the cortical layer. Etiolated shoots, 

 exposed to full sunlight, were severely injured at first. There was a 

 breaking down of both epidermal and interior cells. After a few days' 

 exposure, however, they soon developed a cuticle and the cortical cells 



