GROWTH AND MODIFICATION OF THE STEM. 



69 



less definite for the species is attained, 

 so that the leaves and branches become 

 separated by uniform vertical as well as 

 circumferential spaces. This brings us 

 to another great distinction between the 

 stem and the root, in which latter we 

 have found a continuous and uniform 

 longitudinal development- The rule that 

 a branch develops in each leaf-axil is 

 habitually departed from in the leaf-rep- 

 resentatives constituting the flower, and 

 accidentally in many other cases. Its 

 failure to develop may be temporary, al- 

 though often very long continued, or it 



may be permanent. Upon the other hand 

 the subtending leaf may fail, accident- 

 ally, or, in a few cases, habitually, to de- 

 velop, so that the branch does not show 

 its axillary nature. Finally, we note that 

 a branch may accidentally, or, in some 

 cases habitually, develop from some other 

 point than the leaf axil, or two or more 

 may develop, at least partially, from one 

 axil, either side by side or in a vertical 

 row. 



Not only may a lateral branch thus fail 

 to develop, but the apical extension of the 

 growing point may fail, accidentally or 

 habitually, the growth being continued by 

 means of one or more branches only. A 

 stem so formed is called a Sympodium or 

 Sympodial Stem, One in which the apical 

 growth is continuous is called a Mono- 

 podium or Monopodial Stem. Sometimes 

 instead of the lateral or terminal growing 

 point failing altogether to develop, it may 



ficial they are called Trichomes. The 

 characters of trichomes upon stems or 

 leaves, particularly the latter, are of the 

 utmost importance in diagnosis. Their 

 study, however, save as to the surface- 

 characters which they collectively pro- 

 duce, pertains to histology. The gross 

 surface character so produced will be 

 taken up in connection with the leaf. 

 When appendages are of deeper origin 

 they are called Outgrowths or Emer- 

 gences. These may contain vascular tis- 

 sue, connected with that of the stem. 

 Outgrowths are, for the most part, in the 

 form of spines, hooks (Fig. 357a), warts, 

 suckers (Fig. 358a*), or grasping organs, 

 Both trichomes and outgrowths may be 

 regularly or irregularly disposed. 



f^'g.356. 



Roots may develop from branches 

 which are subterranean, resting upon the 



develop in the form of some entirely dif- surface of the ground or high above it. 

 ferent body from an ordinary stem, its rj^j^^ latter may descend and enter the 



growth in such cases usually ceasing per- 

 manently with the perfection of such an 

 organ. When either of these results at- 

 tends the terminal point, the one or more 

 lateral branches may retain their lateral 

 or oblique direction, or, if one, it may be- 

 come erect and take the position nat- 

 urally belonging to the main stem. If 

 then the latter exists in a modified form 

 it will be distinguished from a modified 

 branch by being on the opposite side of 

 the stem from the leaf, instead of being 

 subtended by such leaf (Fig. 356, a the 

 main stem, b the branch). Besides modi- 

 fied or unmodified leaves or branches, 

 stems may develop various other appen- 

 dages. When these are merely super- 



ground, fix themselves to a neighboring 

 body for sustenance or support, or both, 

 or extend into the atmosphere. They 

 may even turn and enter a diseased '"- de- 

 caying portion of their own plant. They 

 normally develop from the node only, but 

 may develop from any other part or even 

 from leaves. 



An undeveloped stem or branch, or the 

 partially developed summit of one, is a 



Bud or Gemma. The bud may be in a 

 process of continuous development of its 



lower elements into mature phytomers, 



with the continuous production of a new 



growing point, or it may pass into a rest- 



♦In the species here illustrated the sucker 

 is a stem, not an outgrowth. 



