220 



PLANT MORPHOLOGY 



consists of a short tuberous stem bearing a few small fleshy leaves that 

 form a cluster around the stem apex (Fig. 175). As a rule, only a single 

 root is present, but sometimes there are two or three roots. In some spe- 

 cies of Lycopodium the sporophyte begins its development as a small 

 tuberous body like that of Phylloglossum. This disappears after giving 

 rise to the ordinary type of leafy stem wdth roots. In other species no 



embryonic body of this kind occurs in the life 

 history. 



Vascular Anatomy. Anatomically the roots 

 and leaves of Lycopodium are essentially like those 

 of other vascular plants. The leaves are only a 

 few layers of cells in thickness and consist of uni- 

 form mesophyll enclosed above and below by an 

 epidermis with stomata. Each leaf has a single 

 median vein. 



The stem structure of Lycopodium is very 

 primitive. A cross section shows an outer cortex 

 and a central cylindrical stele. Because of the 

 absence of a pith, this type of vascular system is 

 a protostele. In some species the xylem forms a 

 star-shaped mass between the rays of which lies 

 the phloem (Fig. 176). In other species, although 

 fundamentally radial, a modification is seen in that 

 the xylem and phloem are somewhat intermixed 

 (Fig. 177), while in still others the two kinds of 

 conducting tissues occur in alternating, transverse, 

 parallel bands (Fig. 178). A protostele that is 

 star-shaped in outline is often designated as an 

 actinostele; one that is circular in outline is called 

 a haplostele; while the type with separate, par- 

 allel plates of xylem is termed a plectostele. The radial type is most 

 primitive and the banded type most advanced. This is shown by the 

 development of the vascular system in a young plant, where, if the banded 

 condition appears, it is always preceded by the radial condition. Also, in 

 many species, the growing stem is radial at the tip and gradually becomes 

 banded farther back. 



All the vascular tissues are primary, there being no cambial activity. 

 The smaller cells at the tips of the radiating arms of xylem are the first 

 elements to lignify, subsequent lignification proceeding toward the center 

 of the stem. Thus the development of the xylem is exarch, the proto- 

 xylem lying external to the metaxylem (Figs. 176, 177, 178). The proto- 

 xylem is composed of narrow^ spiral, and annular tracheids, the metaxylem 

 of larger, scalariform tracheids. 



Fig. 175. Fertile plant 

 of Phylloglossum drum- 

 mondii, twice natural 

 size; sir, strobilus; /, 

 leaves; r, root; tl, pri- 

 mary tuber; t2, secondary 

 tuber. 



