THE USES OF LEAVES. 
97 
usually in very small proportions — about 2 per cent. — a large amount of 
water has to be got rid of. It is therefore a true vital function, and un- 
like evaporation, which is due to heat and occurs with non-living 
substances. 
As transpiration is the throwing off water absorbed by the roots, 
and principally under the influence of the red and violet rays, it has 
be3n thought that leaves could not absorb water. This, however, is 
practically refuted by all the experiences of florists, who invariably 
sprinkle the leaves of cut flowers to keep them fresh. Under ordinary 
circumstances when a plant is freely transpiring and has a good supply 
to draw upon in the soil, it has no need, nor is it capable of taking in any 
more than it can hold ; so that experiments with such plants yield 
negative results. But a very striking proof that they can and do absorb 
water is easily seen if a number of cut shoots of various sorts of herbs 
and shrubs be made and left in the sun for two or three hours until they 
are flagging. Then they must be weighed and laid out on the ground 
during a dewy night. In the morning, if all the superficial moisture be 
carefully removed by a soft cloth, they should be again weighed, and they 
will be found to have gained enormously in weight. Moreover, they will 
have lost all their limp appearance and become as fresh -looking as when 
first cut off. Hence, although they may refuse to absorb water when in 
full vigour, they readily do so as soon as transpiration has exceeded the 
supply by absorption by the roots. 
Last comes the fall of the leaf. When leaves have done their work 
for the season they fall off' : if in one year the trees are called deciduous. 
But if they remain on for a longer period they are called evergreens. 
The process is as follows. First, everything that can be of use to the 
plant is withdrawn from the leaf, the chlorophyll decomposes and passes 
into compounds of tannin, becoming yellow and brown. The protoplasm 
undergoes fatty degeneration," passing into oil &c., after yielding up 
its nitrogenous materials. Nothing but what is useless, such as the 
skeleton &c., is left behind. Then is formed the " absciss " layer of cells 
which cuts the base of the leaf-stalk across, and a puff of wind is suf- 
ficient to detach it. 
This so-called absciss-layer is really nothing more than a continuation 
of the cork-forming layer of the stem. This is constantly being formed 
all over the stem of the tree. But on the first year's twig, which carries 
the leaves of the year, it is intercepted at first where the leaf- stalk issues 
from the twig ; for the woody bundles, liber, &c. are in continuation from 
the twig into the leaf -stalk. But by the end of the season the " phel- 
logen," as the active bark-layer is called, invades the region whence the 
stalk proceeds and gradually cuts off the vascular connection between the 
stalk and the twig. . 
Having thus thrown oft' the leaf the phellogen continues to cover the 
stem helow the cicatrix with cork, which soon obliterates all trace of the 
spot where the leaf was inserted. 
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