104 



some hours a lighted taper is introduced into the jar, the 

 taper may be extinguished owing to the exhaustion of the 

 oxygen and the accumulation of carbon dioxide. In such seeds 

 which contain no chlorophyll there is no carbon-assimilation, 

 but respiration, nutrition, and growth are still actively going 

 on at the expense of the food materials stored in the seeds. 

 The fact that seeds can germinate and that seedlings can develop 

 and attain considerable dimensions in the dark, when no carbon- 

 assimilation is possible, clearly shows that nutrition is by no 

 means the same thing as carbon-assimilation. In the case 

 of such seedlings which have developed in the dark, nutri- 

 tion and growth have taken place at the expense of the organic 

 food materials contained in the seed and the dry weight of such 

 a seedling is found to be actually less than that of the seed 

 from which it sprang. These substances have in fact been lost 

 in respiration, the process which has supplied the energy 

 necessary for nutrition and growth. If on the other hand 

 a seedling is allowed to develop normally in the sunlight, green 

 leaves are produced, carbon is assimilated, and the dry weight 

 of the plant is soon considerably in excess of that of the seed 

 from which it sprang, owing to the fact that more organic 

 substance has been manufactured in the leaves than has been 

 required for respiration. 



Growth. 95. During the growth of cellular 



plants the actual enlargement of the cells is effected more by 

 increasing the actual surface-area of the cell- walls than by in- 

 creasing the substance of the protoplasm, although the latter 

 does of course take place. This enlargement of the cell-walls 

 can only take place provided that the cells are in a state of 

 turgidity and hence a liberal supply of water is essential for the 

 growth of such plants. Like other vital phenomena, growth is 

 only possible within a definite range of temperature. When the 

 latter falls below C, or rises above 40 or 50 C, growth 

 as a rule becomes impossible, while the optimum tem- 

 perature for growth usually lies between 22 and 37 C. 



Light retards growth and plants which have developed 

 in the dark as a rule have unusually long internodes, while 

 their tissues contain more water and their cell-walls are thinner 

 than would have been the case if the plants had been exposed 

 to the sunlight. As carbon-assimilation is impossible in the 



Ligkt. 6C y dark, even if leaves were developed they would be unable 

 to perform their essential functions and plants which have 

 developed in the dark as a rule form no green chlorophyll. 

 Moreover in such cases material and energy are not 



C?*' 



