TISSUES AND SIMPLE ORGAN'S. 



by intercellular spaces. Little is known concerning these organs in 

 monocotyledons. Among dicotyledons and gymnosperms they 

 originate from the cork-cambium, and consist either of entirely su- 

 berized cells or sometimes also of such as are not suberized ; both 

 cell- forms contain intercellular spaces radially arranged correspond- 

 ing to their succcession in development. 



Very frequently lenticels are sufficiently large to be seen by the 

 naked eye, for example, in the birch. Their permeability to air- 

 has already been referred to. Their origin does not always coin- 

 cide with the position of a breathing-pore ; very frequently lenti- 

 cels are formed after bark-formation has begun. (KLEBAHN, who 

 continued the investigations begun by STAHL and others, has studied 

 lenticel-f or mations more particularly.) 



VIII. THE FUNCTION OF EOOTS. 



We shall discuss: 1, the activity of ordinary or subterranean 

 roots, and 2, that of aerial roots. Their internal anatomical struc- 

 ture (the transit-cells in the protective sheath, etc.) has already 

 been discussed. 



(a) Subterranean Roots. 



From the fact that the absorption of food is to be accomplished 

 by closed cells it is easy to comprehend that such food-substance 

 must be in a soluble (capable of osmosis) form. Water and watery 

 solutions of mineral substances whose chemical composition and 

 nature will be considered elsewhere are of special importance. 

 The portion of the root which serves the purpose of taking up the 

 food- substances is comparatively small; it is located behind the 

 root-cap and is considerably increased in surface by its numerous 

 root-hairs. In young roots the portion bearing the root- hairs 

 comprises, in general, the greater portion of the entire root-surface 

 exclusive of the very tip, in older roots only the portion immedi- 

 ately behind the root-tip (see Fig. 80). 



Transverse septa are wholly wanting in the root-hairs, branch- 

 ing rarely occurs, so that they represent long papillose outgrowths 

 of the epidermal cells (Fig. 81). The absence of transverse 

 septa, the thinness of the cell-wall, the irregular curvatures all 

 serve the specific purposes of root-hairs, namely, to bring them in. 



