ORGANS AND SYSTEMS OF ORGANS. 



169 



their succession. The position and arrangement of young organs 

 are also different from their later position and arrangement (final or 

 definitive position). 



There are not less than seven different places of origin for the 

 various organs of the more highly organized plants. They are as 

 follows : 



1. The epidermis. In it the tric/wmes, also the " emergences," 

 with the aid of more deeply seated layers, have their origin. 



2. The apical portion of the stem gives rise to branches. 



3. The terminal portion of roots gives rise to secondary roots 

 (dichotomy). 



4. The meristem (formative tissue) of the stem-organ immedi- 

 ately below the apex gives rise to leaves. 



5. The tissue in the axils of leaves gives rise to axillary shoots. 1 



6. The cambium and all other meristematic tissues within the 

 epidermal layer of various organs, such as stem, leaf, and root, give 

 rise to adventitious branches and roots. 



7. The pericambimn of the root gives rise to the normal root- 

 branches. 



Adventitious formations (6) and normal root-branches (7) origi- 

 nate endogenously, as opposed to the exogenous origin of organs 

 mentioned under 1-5 inclusive. 



From the various groups of cellular plants we shall select the 

 following cases for discussion : 



(a) Among many algae branching of the thallome proceeds 

 from the apical cell ; well exemplified in FlorideoB (Fig. 98, dia- 

 gramatic). 



(b) In some algae (Cladophora, 

 CharacecB) the branches proceed from 

 certain body-cells (Fig. 99). It may 

 be that any or all body-cells can de- 

 velop branches, or it may be that 

 only certain special cells have that 

 power. 



(c) In mosses the conditions are 

 quite different. At the apex of the 

 stem of the moss there is, as a rule, a 



FIG. 98. 



two-edged " or a three-sided 



1 Among angiosperms there is, as a rule, a young shoot for each leaf-axil ; in 

 gymnosperms this is not the rule (this may readily be observed in Taxus and 

 other conifers). 



