254 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



the absence of carbon dioxide. Thus, light of the shorter wave-lengths, and not 

 the possibility of photosynthesis, is the requisite condition for normal form. 

 This conclusion is also supported by the experiments of Vines, ^ who grew plants 

 in Ught but in a soil without iron. Chlorotic plants were thus obtained but 

 their form was quite similar to that of normal plants, even though they were 

 without chlorophyll and, consequently, could not assimilate carbon dioxide. 



Only in certain plants is the shape of the leaves determined by the occurrence 

 or non-occurrence of photosynthesis. Some etiolated leaves, such as those of 

 wheat, contain little protein material and relatively large amounts of carbohy- 

 drates, while some other leaves, such as those of bean and lupine, are rich in 

 protein material and contain almost no carbohydrates at all, excepting only a 

 very little starch in the stomatal guard cells. The relative amounts of proteins 

 in wheat and bean leaves, in the etiolated and normal condition, are given below, 

 in percentage of total green weight. It thus appears that etiolated bean leaves 



Green Etiolated 



Wheat leaves i 99 ± . 28 



Bean leaves 4.9S 8.38 



contain more protein than do normal leaves, but they nevertheless remain small 

 and undeveloped. As has been stated (page 208), the respiratory activity of 

 etiolated bean leaves is very low but respiration is greatly increased when sugar 

 is supphed. Carbohydrates are necessary for the growth of all leaves, but in 

 those of the bean and similar plants, where carbohydrates do not accumulate, 

 these substances must, under normal conditions, be derived directly from photo- 

 synthesis. Thus bean leaves kept in darkness are deficient in carbohydrates 

 and so cannot grow. Leaves of the other group of plants (such as wheat) 

 are not dependent for their supply of carbohydrates at any particular time upon 

 the rate of photosynthesis, for these substances accumulate in such leaves and 

 the latter always contain much starch. Therefore wheat leaves, as has been 

 seen, attain their usual size in darkness, or even become larger than in light. 



The necessity of carbohydrates for normal leaf development is also shown by 

 the experiments of Jost,^ who obtained etiolated leaves of almost normal size 

 in darkness, by supplying the needed nutrient materials. These etiolated leaves 

 lived a long time in spite of the absence of light. When green leaves were placed 

 in darkness, however, they degenerated rapidly, in spite of the fact that nutrient 

 materials were supplied as in the other case. Jost suggests that perhaps the 

 chlorophyll (or the whole photosynthetic apparatus) is subject to decomposition 

 in darkness, thus giving rise to products that may be injurious to the cells in 

 other ways. 



Etiolated plants in darkness give off water at a lower rate than do green 

 plants in hght and, as has been mentioned, the decrease in transpiration rate 

 brought about when plants are kept in a nearly water-saturated chamber exerts a 



' Vines, Sydney Howard, The influence of light upon the growth of leaves. Arbeit. Bot. Inst. Wuraburg 

 2: 114-132. 1882. 



' Jost, Ludwlg, Ueber die Abh&ngigkeit des Laubblattes von seiner Assimilationsthfttigkeit. Jahrb. 

 wiss. Bot. 27: 403-480. 189s. 



