FOLIAGE AND SCALE LEAVES. 71 
not for the vigorous ascending currents facilitated by transpira- 
tion. Transpiration causes the pressure within the trachez to be 
least nearest the leaves, towards which, therefore, the sap natur- 
ally flows. Again, the formation of organic substance, to any 
extent, means not only a plentiful supply of carbon dioxide and 
water, but also of the simple salts dissolved in water. Yet the 
amount of these is so small in the liquid absorbed by the roots 
from the soil that it has been compared to ordinary drinking- 
water. It is necessary, therefore, in order that enough of these 
compounds may be obtained, for a very large quantity of water to 
be absorbed by the root, much more, in fact, than is retained in 
the plant. Where, as in cacti and similar forms, this process is 
very sluggish, little organic matter can be formed and growth is 
extremely slow. 
Saprophytes and parasites, getting, as they do, organic com- 
pounds ready prepared, may dispense partly or entirely with 
chlorophyll. In the latter case the leaves are much reduced in 
size (broom-rape, &c.), or even entirely absent (dodder). 
Insectivorous plants (cf. pp. 54, 61) lay themselves out for the 
capture of animal food, although they are abundantly provided 
with chlorophyll and can thrive fairly well without it. In the 
pitcher-plants a sugary substance is secreted at or near the 
orifice, and in some cases, as in Sarracenia variolaris, there is 
even a sugary track leading up from near the ground. Insects, 
especially ants, are thus attracted, and if they venture to set foot 
on the slippery inner side of the pitcher, no efforts can save them 
from sliding down into the liquid within, where they are drowned. 
In the case of these and the other insectivorous plants the proteid 
substances of the prey are brought into solution by a digestive 
excretion poured out from innumerable glandular hairs or emer- 
gences. ‘The solution then diffuses into the interior of the leaf. 
The excretion closely resembles in composition and function the 
gastric juice of an animal’s stomach. 
Leaves, like roots and stems, carry on the function of respira- 
tion, but it is only easy to detect this in the dark, since in the 
light the taking up of oxygen and giving out of carbon dioxide 
are hidden by the exactly opposite process involved in assimi- 
lation (cf. p. 68). 
Vegetative reproduction by shoots has already been spoken 
of (p. 43). 
The leaf exhibits various forms of motility. Protoplasmic 
movements may sometimes be observed in the cells of the 
mesophyll. This is the case, for example, in Vallisneria spiralis, 
an aquatic form commonly grown in fresh-water aquaria, where 
the protoplasm, carrying with it the chlorophyll granules, may 
