278 TRANSPIRATION. 



the current, which have the power of invigorating the stream, of hastening it if 

 necessary, and also of lessening it under certain circumstances. Also it is arranged 

 that in case of need fluid nourishment in the higher regions of the stem may reach 

 the leaves hy side paths. 



The cells which by means of the exhalation of aqueous vapour into the atmos- 

 phere originates the transpiration-current are, as already mentioned, not far from the 

 terminations of the sap-conducting apparatus. In some mosses they are freely ex- 

 posed to the air. In the Polytrichacese and several other mosses (Barbula aloides, 

 ambigua, rigida) they form short chains of cells like strings of pearls, or bands 

 projecting from the grooved concave upper surface of the tiny leaves (see fig. 61 2 ). 

 Again, among the liverworts are forms, e.g. Marchantia polymorpha, which contain 

 large characteristic air-chambers in the body of their green leaf -like thallus (fig. 

 61 1 ). On the floor of this chamber are green cells which are so grouped together 



Fig. 61. — Transpiring Cells. 



1 Vertical section through an air-chamber of the Liverwort Marchantia polymorpka; X300. 2 Vertical section through 



a leaf of the Moss Barbula aloides; x380. 



as to remind one of the shape of the Prickly Pear (Opuntia). These green cells are 

 thin-walled, and it is from them that water is evaporated. They are not quite 

 freely exposed, like those of the mosses mentioned above, since the roof of the 

 chamber, composed of transparent cells, is extended over them; a chimney-shaped 

 passage, however, is left open through the roof of each chamber by which the water- 

 vapour given off from the opuntia-like cells can escape. These Marchantias furnish 

 a transitional form between the freely exposed transpiring cells on the upper surface 

 of the leaf of the moss and those of flowering plants. In flowering plants the 

 transpiring cells are situated as a rule in the interior of the green leaves, and also 

 in the green cortex of leafless branches, forming a part of that green tissue which 

 has been termed chlorenchyma, or when in the leaves, mesophyll. 



Leaves may be described as consisting of cells filled with leaf -green, or chloro- 

 phyll, placed closely together and joined into layers above one another so as to 

 form a soft mass of tissue containing abundance of sap; this green tissue pierced 

 by the branched water-conducting strands whose ultimate divisions terminate in the 

 tissue mass; the whole surrounded and shut in by a firm cuticle which is perforated 

 in many places by stomata. Cellular passages are also regularly arranged for the 

 purpose of conducting away the organic materials manufactured in the green cells, 

 whilst groups of cells for the support of the whole, serving as beams, strengthening 

 props, and the like, are placed at definite points. 



