LIFE HISTORIES OF FAMILIAR PLANTS 



by being spread out at night-time, it is a very 

 useful device on the part of the plant to close 

 them together at nightfall ; for then they are 

 kept warm and their tissues are protected from 

 the chilly night air. If the leaves were fully 

 expanded, they would probably accumulate mois- 

 ture, and at the slightest approach of cold or frost 

 receive a chill which might cause them serious 

 damage. However, with leaflets folded closely 

 together, both in the case of the sensitive plant 

 and the oxalis, rain drops and moisture are 

 conducted to the earth below. So the first oxalis 

 plant, which, in the natural variation of living 

 things (for no two organisms, and no two habits in 

 an organism, are identical) adopted the habit of 

 drooping its leaves slightly when the temperature 

 was lowered, found that it was beneficial, and it 

 forthwith conveyed the hint to its race ; and the 

 habit, being good, it became hereditary. Thus the 

 wood-sorrel, and others of its genus, acquired the 

 habit of sleeping leaves ; which, of course, 

 represents sensitiveness to external conditions, 

 such as light and temperature. 



The species shown in Fig. 49 (Plate 32) has 

 brown or copper-coloured foliage, and the leaves 

 of the common wood-sorrel also often develop on 

 their under sides this same reddish hue. This 

 colouring matter, as I have previously pointed out, 

 has the peculiar propert}^ of utilising the rays of 

 light and converting their energy into heat, which 

 naturally benefits the growing plant. For the 

 same reason the buds of many plants, when they 



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