MORPHOLOGY OF THE ANGIOSPERMS 



The carpel is still in process of closing; some stages are to be seen in 

 living taxa. The vessel has arisen independently many times within the 

 angiosperms, and all stages of its development are found in living 

 angiosperms. A supposed basic difference in leaf-trace number be- 

 tween angiosperms and gymnosperms — angiosperms with an odd, 

 gymnosperms with an even number — has been shown to be invalid. 

 But the angiosperms, though not set apart by any single character, 

 clearly represent a well-defined stock, distinct from other seed-bearing 

 plants. 



The sporophytic body of angiosperms shows greater variety in form 

 and size than that of any other major plant taxon, ranging in habit 

 from tree to herb; in size, from the minute Wolffia to the tallest Euca- 

 lyptus; in form, from the simplicity of a thallus to the complex branch- 

 ing of trees and giant vines; in flower structure, from the simplicity of 

 a single, naked sporophyll to the complexity of organs of four types, 

 with much connation and adnation. The gametophytes, although con- 

 sisting of rather few cells, likewise vary a great deal, especially in cell 

 number and arrangement. 



Structurally, the plant body consists of an axis, branched or un- 

 branched, with lateral appendages.* The axis is commonly divided on 

 structural and functional grounds into stem, with appendages and 

 endarch primary xylem, and root, without appendages and with exarch 

 primary xylem. Stem and root may form a continuous axis, as in most 

 seedlings, with a structurally transitional section, the hypocotyl; or roots 

 may develop as appendages of the stem. Continuity of root and stem 

 in the embryo and seedling of the higher plants has been, in part, the 

 basis for the theory that these two organs represent the specialized parts 

 of an original primitive axis which constituted the entire sporophyte, as 

 in simpler members of the Psilophytales. 



The stem alone has been considered to represent the entire primitive 

 axis, with the root a secondarily developed organ. This concept is based 

 on the endogenous origin of the radicle and on the common origin, in 

 some embryos and seedlings, of roots as major appendages of the base 

 of the stem. Evidence in support of this view has been seen in the 

 absence or nonfunctioning of the taproot in some supposedly primitive 

 Liliales, where the entire root system is formed by adventitious roots. 

 (Under this theory, the absence of a taproot has been considered more 

 primitive than its presence.) But absence or nonfunctioning of an em- 

 bryonic taproot seems to be a derived condition. The vascular struc- 

 ture of the hypocotyl, in its symmetry and relations to the primary 

 vascular skeleton of stem and root, supports the view that the primary 



* Anatomical details, already discussed in Eames and MacDaniels, Introduction 

 to Plant Anatomy, are herein largely omitted. 



