FOLIAGE AND SCALE LEAVES. 7 1. 



not for the vigorous ascending currents facilitated by transpira- 

 tion. Transpiration causes the pressure within the tracheal 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 ckloroplryll 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 Sarracema 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 



