70 STUDY OF COMMON PLANTS. 



The internal structure of tlie great majority of leaves 

 is essentially the same as we have seen in the English ivy. 

 Fibro-vason- The midrib and veins, composed of fibers and 

 lar bundles, tracheids, present a strong frame-work by means 

 of which all the parts are supported, and which also serves 

 as the conducting system of the leaf. The green parts 

 consist of chlorophyll-bearing, parenchyma cells, the chief 

 function of which is the manufacture of organized food 

 substances. An extended comparison of the leaves of 

 Assimilating many species of plants shows several interesting 

 oells. arrangements for bringing the assimilating cells 



into an advantageous position as regards the light. In the 

 first place, the leaf itself " turns towards the light," i.e. 

 places itself so that the upper surface is perpendicular to 

 the incident rays. In the second place, the palisade cells 

 are themselves nearly perpendicular to the leaf surface, a 

 position in which their contents are brought into relation 

 with the light, without, however, cutting it off entirely 

 from the cells below. Finally, the chlorophyll bodies 

 vary their position in the cells according to the intensity 

 of the light, ranging themselves so as to expose as large a 

 surface as possible when the illumination is feeble, and a 

 less surface when it is too intense.^ In addition to these 

 arrangements with reference to light, the assimilating cells 

 are grouped in such a manner as to facilitate the convey- 

 ance of water to them by the fibro-vascular bundles, and 

 the removal of elaborated food substances through the 

 same channels.^ 



It is thus seen that the leaf is an extremely delicate 

 organ, adapted to the performance of certain important 

 functions. Their first and most characteristic function 



iSaclis, I.e., p. 617 et seq. 



2 Haberlandt, Physiologische Pflanzenanatomie, p. 184 et seq. 



