190 



BOTANY 



nutrient salts is present in the transpiration current ; as in that ease if, through 

 continued evaporation, the nutrient water should become too concentrated, it 

 might act disastrously" upon the plant. Alkalies usually tend to increase turgidity, 

 while acids diminish it. 



It has already been pointed out, in describing the morphology of 



Fig. 171). — Stoma of Helleborus sp. hi transverse section. The darker lines show the shape 

 assumed by the guard-cells when the stoma is open, the lighter lines when the stoma is closed. 

 (After Schwendener.) The cavities of the guard-cells with the stoma closed are shaded, and 

 are distinctly smaller than when the stoma is open. 



the stomata, that they are chiefly to be found on the surfaces of the 

 leaves. The leaves are accordingly to be considered as special 

 organs of transpiration (and assimila- 

 tion, p. 196). This is also evident from 

 the manner in which the vascular bundles 

 branch after entering the leaves. ' As a large 

 water-main divides into a network of smaller 

 pipes where the consumption of the water 

 takes place, so a leaf-trace bundle, after its 

 long and uninterrupted course through the 

 stem, suddenly branches as soon as it enters 

 the leaf-blade. The adjoining illustration 

 (Fig. 178), showing the nervature or dis- 

 tribution of the vascular bundles in a 

 Crataegus leaf, will convey some idea of the 

 extensive branching which the bundles of a 

 leaf undergo, especially when it is taken 

 into consideration that only the macro- 

 scopic and none of the finer microscopic 

 branchings are represented in the figure. 

 By means of this conducting system, a 

 copious supply of nutrient water can be delivered directly from 

 the roots to every square millimetre of the leaf. There is, how- 

 ever, a special reason why the leaves are so abundantly supplied. 

 They are the actual laboratories of plants, in which, out of the 

 carbonic acid of the atmosphere and the water, and nutrient salts of 



Pig. 177. — Stoma of a perianth- 

 leaf of Galtouia candicans. -S, 

 Guard -cell with diminished 

 turgidity, having the wall on 

 the side towards the opening 

 straight ; S', turgescent guard- 

 cell with curved lateral wall, 

 half opening the passage i. 

 (After Leitoeb.) 



