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 allowed 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 
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 B, in response to 
warmer conditions, the 
leaves have spread apart 
and have become freely ex- 
posed. After WARMING. 
