MORPHOLOGY 



111 



no sharp distinction between primary cortex and central cylinder, com- 

 parative investigation alone can determine whether a tissue belongs to 

 the primary cortex or to the central 

 cylinder. Although the fundamental 

 tissue of the primary cortex is pre- 

 eminently a chlorophyll -containing 

 tissue, portions bordering on the 

 epidermis frequently become con- 

 verted, for mechanical purposes, into 

 strands of collenchyma or scleren- 

 chyma. Such a mechanical tissue, 

 which serves to strengthen the epi- 

 dermis, is known as a hypoderma. 

 Of the tissues composing the central 

 cylinder, the pericycle, the primary 

 medullary rays, and medulla consist 

 of fundamental tissue, and are chiefly 

 composed of colourless parenchyma. 

 A part, however, of the tissue of the 

 pericycle may become sclerenchy- 

 matous (Fig. 125, pc) ; sclerenchy- 

 matous elements also often surround 

 individual bundles as sheaths, or accompany the phloem portion in 

 the form of strands (Figs. 117, 119). Whenever such a sheath of 

 sclerenchyma is developed about a bundle, it is always interrupted on 

 both sides of the bundle, at the junction of the xylem and phloem 

 portions, by parenchymatous cells, or by cells which are only slightly 

 thickened and lignified. These cells facilitate the exchange of water 

 and food material between the vascular bundles and the fundamental 

 tissue, and are spoken of as transfusion strands. 



Fig. 127. — Transverse section of the rhizome 

 of Pteris aquilina. s, Vascular bundle 

 strands (schizosteles) ; I, sclerenchymatous 

 plates ; lp, peripheral zone of sclerenchy- 

 matous fibres ; R, cortex ; e, epidermis. 

 (XV.) 



In the case of Phanerogams the central cylinder is simple and occupies a more 

 or less central position. In some few instances, however, it breaks up into several 

 partial cylinders or schizosteles. Such schizosteles are found in the stems of 

 Auricula and Gunnera. The tissue that surrounds and separates these central 

 cylinders corresponds to the primary cortex. 



The vascular bundle strands (p. 104) of the Pteridophytes are also to be regarded 

 as schizosteles. In the stems of Ferns they are usually separated from one another 

 (Fig. 127) and situated in the fundamental tissue belonging to the primary 

 cortex. This is also the case in the Selaginellas. In Lycopodium (Fig. 128), on 

 the contrary, the schizosteles become united into a central gamostele. In these 

 o-amosteles the xylem portions of the single schizosteles form separate bands, whilst 

 the alternating phloem portions are fused with one another. The vascular bundles 

 of the Equisetaceae (Fig. 346), on the other hand, are collateral. They are similar 

 to those of Phanerogams, and, like them, are arranged in a circle within the central 

 cylinder (cl) with the xylem innermost and the phloem outermost. They sur- 

 round a large medullary cavity {in) formed by the disruption of the pith of the 

 intemodes. The central cylinder is completely enclosed by the primary cortex (ch). 



