ORGANS AND SYSTEMS OF ORGANS. 167 



gences. The trichomes are not formed with auy regularity or 

 according to any systematic arrangement. Epidermal trichomes- 

 differ greatly in form (see Epidermal System, p. 53). 

 The two following organs 



3. Caiilome (stem-organ) and 



4. Phyllome (leaf-organ) are so intimately related that they 

 must be treated together. The stem (caulome) is really the central 

 organ which bears leaves along the sides below the apex. The 

 leaves are lateral organs on the apex and sides of the stem and its 

 branches which are not irregularly formed here and there, but, in 

 general, are developed acrojjetally, that is, from the base toward the 

 apex (see Fig. 101). It is impossible to find any fundamental dif- 

 ferences between the internal and external structure of stem and 

 leaf. It is true leaves usuallj- present an expanded surface, but 

 there are likewise flat stems {Cactacem) and cylindrical leaves 

 {(Jon{fer(B). 



5. The root is the organ whose cell-forming apex is covered by 

 a protective tissue, the root-cap, and which never bears leaves. In 

 contrast to trichomes and phyllomes, the \'ooi()iQv^o\^%endogenously, 

 so that it must force its way through some tissue before it can come 

 to the surface. (The term "rhizome" does not have the same 

 meaning as root, as we shall learn later.) 



B. Modification of Orgajsts. 

 («) Modifications of Stem and Root. 



Certain modifications of the caulome, due to its subterranean 

 position, are of special physiological importance. Such eaulome- 

 organs are without foliage-leaves, or flowers. These modifications 

 as well as a few root- forms will now be briefly discussed. 



The following are the subterranean stem-modifications, of which 

 there may be intermediate forms : 



{a) The rhizome or root-stock — stem- and leaf-organ moderately 

 developed. 



(b) The tuber — stem enormously developed, leaves very small. 



(c) The balh — stem small, leaves very large. 



At this point we shall introduce a biological classification of 

 plants; that is, a classification derived from the life-processes of 

 plants. 



