CH. XIIl] PLANTS IN DARK AND IN LIGHT 129 



chlorophyll-corpuscles in the cells of the leaf, as we have 

 seen. 



The principal features of the process may be easily 

 demonstrated as follows. A number of Mustard seeds 

 are divided into four equal packets. Three of these 

 portions are sown in as many pots of fine soil, and the 

 other portion is placed aside. When the seedlings appear 

 above ground in the three pots, one of the latter is exposed 

 to full daylight close to a window, one far back in the 

 room in faint diffuse light, and the third in complete 

 darkness. 



In a few days the plants exposed to full light will be 

 found to be growing vigorously, their leaves deep green 

 in colour, and evidently developing into normal plants. 

 Those in the faint light will be " drawn" and "sickly" 

 (to use the gardener's phrases) and obviously less vigorous 

 as regards thickness of stems, size and depth of colour of 

 leaf. Those in the dark will be very pale and watery, 

 and much drawn (etiolated), and are evidently very weak 

 and will soon die. 



If we compare the dry weight of each of the three sets 

 of seedlings, with the dry weight of the fourth packet of 

 seeds which had been kept back for the purpose, we shall 

 find that while that of the plants in diffuse light is con- 

 siderably greater than that of the plants in the dark, it is 

 decidedly less than the dry weight of our control seeds ; 

 whereas the dry weight of the plants fully exposed to the 

 light will be greater than that of the seeds themselves. 



In other words, the plants fully exposed to light have 

 decidedly gained in dry weight : those in darkness have 

 lost substance : and those in the feeble light have just 

 about maintained their weight. Now what does this 

 mean ? 



Careful analyses of thousands of such cases prove that 

 w. ii. 



