4 MORPHOLOGY OF THE ANGIOSPERMS 



morphological evidence — into the leaf-skin theory (p. 21). The third 

 theory took several forms, including the phyton theory and "phytonism" 

 (p. 21). (The term phytonism has also been applied to the theory that 

 the stem consists of leaf bases. ) 



If the origin of the complex sporophyte of the higher vascular plants 

 is to be seen in the simple body of the Psilophytales, the description of 

 the body of seed plants as made up of two basic parts, root and shoot, 

 seems reasonable. In general method of development — development by 

 apical meristems — they are alike, but they differ in basic vascular struc- 

 ture, in method of later increase in length, and in structure of the outer 

 tissues. The intercalary growth of stems and leaves (origin of the 

 petiole) is absent in roots. Histological elaboration of the outer tissues 

 in stems is chiefly in the formation of a complex cortex; in roots, of a 

 specialized epidermis with root hairs. In the angiosperms, the presence 

 of an endodermis has been considered a root character, distinguishing 

 root from stem, but an endodermis is present in stems of many taxa — in 

 rhizomes, in seedlings, in aquatic plants, and in some parts of the stems 

 of the majority of herbs. And there is excellent histological evidence in 

 other taxa that the endodermis — characteristic of the entire plant body 

 in many pteridophytes and probably present in the stems and leaves of 

 ancestral angiosperms — has largely been lost. The endodermis ties to- 

 gether root and stem as parts of a continuous plant body, and its pres- 

 ence is not evidence that an organ is a root. 



Embryo structure has been seen to demonstrate the unity of the axis 

 in embryos that have, in the hypocotyl, the radial vascular structure of 

 the root and continuity of the endodermis in the hypocotyl and 

 the epicotyl. This continuity of the endodermis is considered a re- 

 capitulation of the evolutionary history of the plant body; the axis of 

 the embryo represents the simple, primitive plant body. In the embryo, 

 in the seedling, and persisting (except in some monocotyledons) in the 

 mature plant, the hypocotyl is the section of the axis transitional 

 anatomically from root to shoot; its origin goes back to the early elabora- 

 tion of the sporophyte as shoot and root. 



The description of the axis of the angiosperm embryo as, in structure, 

 a continuous organ, root at one end, stem at the other, has been con- 

 sidered morphologically inaccurate; the embryos of most monocotyle- 

 dons and those of some dicotyledons cannot be so interpreted. The 

 plumule of these embryos is commonly described as "lateral," but the 

 lateral position is only apparent; the plumule is morphologically termi- 

 nal, as in the dicotyledons (see Chap. 9). 



The Root 



In the angiosperm embryo, root and shoot form a continuous struc- 

 tural axis; in the pteridophytes, the embryonic root is the first of a series 



