108 



DICOTYLEDONS 



keep pace with its expenditure. This happens when the plant 

 ripens its numerous seeds, each of which requires rich food. It 

 can be observed that the aroma of the plant is less at the time 

 of [lowering and fructification. 



Plants growing in the shade 

 have much less of this volatile oil 

 than plants growing in open places. 

 The shade compels the plant to 

 enlarge its foliage, and thus neces- 

 sitates a much larger expenditure 

 of food stuff, so that much of it 

 cannot be stored up for the future. 

 Such plants do, therefore, not 

 flower so readily as plants in 

 open places. 



In addition to the glandular 

 hairs, the surface of the whole 

 plant is covered by woolly hair. 

 This coat of hairs reduces evapo- 

 ration by interfering with the free 

 circulation of air on the surface 

 of the leaf, which we shall under- 

 stand from a little experiment. 

 Moisten two sponges of equal size 

 and put them on the same place for 

 drying, l)ut wrap a })iece of cloth 

 round one of them. We shall tind 

 that the covered sponge keeps its 

 moisture longer than the other. How does this happen V From 

 both sponges water-vapour rises, but the vapour under the cloth 

 cannot escape so freely as from the uncovered sponge, and so 

 the rate of evaporation is slackened. Precisely the same happens 

 with two leaves of which one is glabrous or uncovered and the 

 other hairy or downy. Excessive loss of moisture through the 

 epidermis would cause the phmt to wither, as it would not be 

 in a position to make ui) for this loss by the sucking action of 

 the roots. 



Fig. 104. — The Tulasi Plant 

 (Ocimum sanctum). 



