308 



BOTANY 



Growth of the Stem. The apex of the stem is usually a broad, much-flat- 

 tened cone (Fig. 273). The centre of this is occupied by a group of small initial 

 cells, from which lateral and basal segments are cut off, apparently without any 

 definite order. From the lateral segments are derived the epidermis and 

 cortex ; from the basal ones, the central vascular cylinder of the stem. Branches 

 may arise laterally, or there may be a true dichotomy of the apex. 



A section of the stem (Fig. 270, C) shows within the epidermis a mass of 

 cortical tissue, which in most species is composed, largely, of sclerenchyma. 

 Bounding the central vascular cylinder is a well-defined endodermis, within which 

 there is a pericycle composed of one or several layers. This tracheary tissue is 

 arranged in plates which are transverse in the horizontal stems, but more or less 

 confluent in the upright shoots, so that the xylem in the latter presents, in cross- 

 section, a stellate appearance. Alternating with the xylem plates, are masses 

 of phloem elements, the rest of the cylinder being occupied by parenchyma. 

 The tracheary elements are for the most part scalariform tracheids, like those of 

 the Ferns. The sieve-tubes are smaller, and not so well developed. 



The Leaf 



The leaves are small, lanceolate, with 

 broad sessile base. The simple median 

 vascular bundle is concentric, but with- 

 out a definite endodermis. Where the 

 leaves are spirally arranged, stomata 

 occur upon both surfaces. In those 

 species with decussate leaves, like L. 

 complanatum, they are formed upon 

 the lower surface only. 



The Root 



Like the stem, the root (Fig. 273, 

 C) in Lycopodium does not show a 

 single initial cell. There are separate 

 initials for each of the primary tissues, 

 Calyptrogen, Dermatogen, Periblem, 

 and Plerome. The first gives rise to the 

 tissues of the root-cap ; the second to 

 the epidermis ; the third to the cortex, 

 and the last to the central vascular 

 cylinder. Branching of the roots is a 

 true dichotomy, the initial tissues at 

 the apex dividing into two perfectly 

 similar groups. The structure of the 

 tissues in the complete root is much 

 like that in the Ferns. The vascular 

 bundle is diarch. 



The Sporangium 



The sporangia (Fig. 270) are kidney- 

 shaped capsules placed singly upon the 

 inner surface of the sporophylls, which 



B 



r---r 



FIG. 274. Lycopodium lucidulum. A, 

 shoot with gemmae, k, and sporangia, 

 sp ( X 2) . B, single gemma ( X 4) . 

 C, germinating gemma of L. selago 

 ( X 4) . ( C, after CRAMER .) 



