THE PROTOPLAST AS HOUSE-BUILDER 



41 



is misleading. Iodine denotes the presence of starch-grains, which often 

 occur but by no means always in the corpuscle. 



Specially interesting is the fact that light is a necessary condition for 

 the formation of chlorophyll. Grow a plant in the dark, and its leaves will 

 be yellow and sickly ; bring it 

 forth to the light, and it will 

 become green and healthy. 

 Hence it will be readily gathered 

 that chlorophyll is seldom found 

 in the roots of plants. The roots 

 of the Common Buckbean or 

 Marsh- trefoil (Menyanthes trifoli- 

 ata) may be cited as a curious and 

 in so far as underground roots 

 are concerned perhaps an 

 unique exception ; but the green 

 aerial roots of some epiphytal 

 Orchids (fig. 64) contain this im- 

 portant substance. The white- 

 ness of celery is due to the 

 exclusion of light from the stem 

 and leaves, which are banked 

 round with earth as fast as they 

 grow. Hindrance is thus offered 

 to the formation of chlorophyll, 

 and by this mode of cultivation 

 the rank coarse taste of the plant 

 is completely removed, and the 

 mild sweet flavour which we as- 

 sociate with table celery is im- 

 parted to it. In its natural state 

 celery is a poisonous plant. 



Doubtless the reader will have 

 noticed how quickly the pale 

 unfolding leaves of spring as- 

 sume their characteristic hue if 

 the weather be bright and sunny ; 

 and, on the other hand, how 

 slowly this change is effected 

 during a succession of dark 



cloudy days. This fact is more remarkable in tropical countries than 

 in England. It frequently happens in America that clouds and rain 

 obscure the atmosphere for several days together, and that during this 

 time the buds of entire forests expand themselves into leaves. These 



FIG. 64. AERIAL ROOTS OF AN EPIPHYTAL 

 ORCHID. 



