ADJUSTMENT TO LIGHT 85 



accomplished by the leucoplasts, and hence can occur only in 

 living cells. The sugar solution is taken up by the leucoplasts, 

 which remove by chemical action a molecule of water from each 

 molecule of the glucose, and deposit the resulting starch in their 

 own substance. This deposition of starch takes place rapidly 

 at night, and slowly, if at all. by da3^ In consequence, the starch 

 grains of storage tissues regularly show layers corresponding to 

 this periodical activity. As the starch grain grows, the leuco- 

 plast is stretched thinner and thinner until it becomes invisible. 

 It never disappears, however, since, at the time when the reserve 

 material is to be used, it must dissolve the starch by again com- 

 bining water with it to form sugar. 



Starch is stored to serve as food material for starting new 

 growth and continuing it until a constant supply of food can 

 be furnished by the new leaves. In the seed, it nourishes the 

 young plant until the latter strikes root and carries its leaves 

 into the light and air. The stored material of root or rootstalk 

 gives a new shoot, while that of the woody stems enables the 

 buds to burst and to send out leaves and flowers. In seeds the 

 reserve material is stored in the endosperm about the embryo, 

 as in the grasses, or it is stored in the leaves of the embryo itself, 

 e.g., in the bean, pea, etc. In perennial plants, storage often 

 occurs in the stem. In herbaceous perennials, the underground 

 stem, rootstalk, tuber, bulb, or conn serves this purpose; in trees 

 and shrubs, the pith and medullary rays of the woody stem 

 itself are used for storage. True roots serve more rarely as store- 

 houses, but in certain biennial and perennial herbs they have this 

 function. 



Experiment 27, Storage tissues. ^lake cross-sections of a wheat 

 kernel, a bean, a woody stem, a rootstalk or tuber, and a root. Stain 

 lightly with iodin, and look for leucoplasts. Make a diagram of each 

 section showing the extent and the location of the storage tissues. 



107. Influence of light upon the number and position of 

 chloroplasts. Only a small amount, usually less than 5' (^, of the 

 light that reaches the chloroplast is used in photosynthesis. The 

 remainder is converted into heat, and produces a vaporization of 

 the water in the plastid. The effect is less pronounced in the 

 shade owing to the lower light intensity. In consequence, chloro- 

 plasts arrange themselves in the sunshine in line with tlu^ light 



