THE PLANT BODY 21 



"dichotomous" branching of a palm is probably not true dichotomy. 

 See Chap. 11.) It has been pointed out in studies of apical meristems 

 that leaf and branch primordia have diflFerent origins; leaf initials arise 

 less deeply in the meristem than do branch initials but this difference 

 is apparently not constant. (Little attention has been given to the 

 origin of branch primordia.) 



The various interpretations of the make-up of the shoot fall into 

 three morphological categories: that in which the stem is regarded as 

 an axis, with lateral lobes or appendages, tlie leaves; that in which the 

 shoot is considered a multiple structure, made up of segments or units 

 called phytons; and that in which the stem is considered a secondary 

 structure, built up partly or wholly of leaves, which are the fundamental 

 units. On the basis of the second and third interpretations, the phyton 

 theory and the leaf-skin theory of the nature of shoot have been pro- 

 posed. 



Akin to the interpretation that the leafbase is, in part, cauline, is the 

 theory that the outer tissues — "skin" — of the shoot axis consist of de- 

 current leaf bases. This "skin" has been described for some taxa, both 

 dicotyledons and monocotyledons, and regarded as present in most, 

 probably in all, angiosperms. Formation of the "leaf skin" is considered 

 to be by downward continuation and development of the leafbase, as 

 the internode elongates. Limitation of the "skin," both externally and 

 internally, has been variously interpreted; according to one descrip- 

 tion, this is "a question solely of definition." Obviously, the theory meets 

 major morphological difficulties where structural limits are not set, 

 and it seems of little importance. 



Under the phyton theory, the shoot consists of "units of growth" 

 that are renewed by a type of terminal "budding." The make-up of 

 these units has been variously defined: as an internode with its attached 

 leaf; a leaf with a root (the internode being the base of the leaf); a 

 segment of the stem, limited by nodes, with or without a leaf; a leaf 

 primordium with its base incorporated in the axis. (A root, as a part 

 of the phyton, is lacking in angiosperms.) Concepts of the shoot as 

 consisting of a series of structural units are old and have been ob- 

 scured by the dominance of the stem-and-leaf theory. Anatomically, 

 units like those described under the leaf-skin and phyton theories do 

 not exist; the shoot is the basic unit. 



The Anatomy of the Plant Body* 



Structurally, the basic plant body is that built up by the embryo and 

 its apical meristems. This is the primary bodi/. All fundamental body 

 parts are represented in the primary body. To this primary body is 



* Anatomy of the flower in detail is discussed in Chap. 3. 



