FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 51 



white clover displaying it. It can be observed that the 

 expanded seed leaves (cotyledons) of many young germinat- 

 ing plants shift their positions at night (see Fig. 43), often 

 assuming a vertical position which brings them in contact 

 with one another,, and also covers the stem bud (plumule). 



Certain leaves with well-developed 

 protective structures are able to en- 

 dure the winter, as in the case of 

 the so-called evergreens. In the 

 case of juniper,, however,, the winter 

 and summer positions of the leaves 

 are quite different (see Fig. 44). In 

 the winter the leaves lie close against 

 the stem and overlap one another; 

 while with the coming of warmer 

 conditions they become widely 

 spreading. 



39. Protection against rain. It is 

 also necessary for leaves to avoid 

 becoming wet by rain. If the water 

 is allowqffl. to soak in there is danger 

 of filling the stomata and interfering 

 with the air exchanges. Hence it 

 will be noticed that most leaves are 

 able to shed water, partly by their 

 positions, partly by their structure. 

 In many plants the leaves are so ar- 

 ranged that the water runs off towards the stem and so 

 reaches the main root system ; in other plants the rain is 

 shed outwards, as from the eaves of a house. 



Some of the structures which prevent the rain from 

 soaking in are a smooth epidermis, a cuticle layer, waxy 

 secretions, felt-like coverings, etc. Interesting experi- 

 ments may be performed with different leaves to test their 

 power of shedding water. If a gentle spray of water is 

 allowed to play upon different plants, it will be observed 



B 



FIG. 44. Two twigs of juni- 

 per, showing the effect of 

 heat and cold upon the 

 positions of the leaves. 

 The ordinary protected 

 winter position of the 

 leaves is shown by A ; 

 while in 5, in response to 

 warmer conditions, the 

 leaves have spread apart 

 and have become freely ex- 

 posed. After WARMING. 



