TRANSPIRATION. 281 



entirely absent. On the upright leaves of flags, asphodels, amaryllis, and various 

 other bulbous plants, and on the vertical leaf -like structures (phyllodes) of the 

 Australian acacias, as well as on some of the needle -like leaves of conifers, the 

 stomata occur on both sides in almost equal number. In the mimosas and various 

 other plants, having, in common with the mimosas, the characteristic faculty of 

 altering the position of their leaflets when stimulated externally, numerous 

 stomata are found on both sides of the leaf. 



Most stomata are elliptical when open; rarely circular or linear. The length of 

 stomates varies between 0'02 and 0'08 mm., the breadth between O'Ol and 0*08 mm. 

 Pines, orchids, lilies, and grasses have the largest stomata; water-lilies, olives, and 

 some fig-trees, the smallest. 



The stomata in the epidermis, the passages and cavities below them into which 

 the thin-walled cells of the green tissue evaporate water, and the strands through 

 which the sap is conducted from the roots to the green tissue, all work in connec- 

 tion with one another like the various parts of a machine. Each portion of the 

 mechanism helps and depends upon the others, the immediate result of the common 

 work being always the elevation of that nutritive fluid which is brought by the 

 absorptive roots into the plant. In the main, therefore, the result obtained by 

 transpiration is the same as that which root-pressure aims at, and it might be 

 thought (taking for granted the truth of the above statement) that either root- 

 pressure or transpiration is superfluous. Or perhaps transpiration and root-pressure 

 work in a complementary manner together. Perhaps the conditions between the 

 two forces are so arranged that the fluid taken in by the absorptive cells from the 

 nutritive soil is forced up to a certain level by root-pressure, and from thence is 

 promoted to still higher levels by means of transpiration? This would suggest a 

 comparison with the raising of water from a spring situated in a valley-basin sur- 

 rounded and shut in by mountains. In the depth of the basin exists underground 

 water which is fed by the subterranean supply coming from the mountains. Ac- 

 cording to the pressure of this supply, the water in the lower earth-strata of the 

 basin rises to a certain height. This pressure is not strong enough, however, to 

 drive the water to the surface of the basin, and in order that it may reach this, it 

 is necessary to employ a pump, which will reach down to that stratum of earth 

 which is saturated by the underground water. But the level of this water 

 differs in summer and winter. It depends also upon the amount of rainfall on the 

 neighbouring mountains, which may undergo great fluctuations. In some years the 

 underground water in the spring has almost risen to the upper opening; at other 

 times only the deepest strata of the valley-basin contain water. The pump, by 

 which the water has to be raised, must be constructed with all these possibilities in 

 view, and must be so regulated that the absorbent action is felt as far down as the 

 deepest position which the underground water is known to take. 



Transpiration behaves in like manner in the portions of a plant above ground, 

 and its action on the fluid food taken in by the roots may be compared with that of 

 a suction-pump. It would be a quite inadequate arrangement if the sucking actioo 



