MORPHOLOGY OF HIGHER PLANTS. 



343 



cotyledons. The central portion of the stele is frequently differ- 

 entiated into a PITH of parenchymatic structure, the cells of which 

 often contain large quantities of starch. In addition in the pitli, 

 we often find the same types of secretory cells or receptacles as 

 occur in the cortex (as in Apocynum). The pith may constitute 

 a homogeneous tissue or be broken, as in Phytolacca, Carya, 

 Halesia, etc., where a longitudinal section shows the pith divided 

 into a row of broad cavities formed by a separation of the cells 

 as a result of the rapid longitudinal growth of the stem. 



Finally it may be mentioned that cork is of frequent occur- 

 rence, especially upon stems that persist more than one year. 

 The cork may arise in the epidermis itself, or it may develop in 

 the hypoderm.al strata of the cortex, or in still other cases we find 

 its development much deeper, even within the pericycle. 



Fig. 197. Schematic representation of different types of mestome strands or fibro- 

 vascular bundles: s, sieve; t, tracheae or vessels; e, position of the earliest tracheae formed; 

 a, radial bundle or mestome strand; b, collateral bundle; c, bicollateral bundle; d and e, 

 concentric bundles. — After Meyer. 



In regard to the increase in thickness, the stem develops much 

 like the root, as in the throwing ofif the peripheral tissues extend- 

 ing from the epidermis to the endodermis, or of the epidermis 

 and adjoining cortex, the displaced tissues are replaced by strata of 

 cork and secondary cortex. The mestome strands in the stem, 

 however, grow in a more regular manner than is the case with 

 those of the root, as is seen in the very distinct and frequently 

 very regular layering of the tissues of woody stems, forming the 

 so-called " Annual Rings," where each ring represents the growth 

 that occurs during a single year. The development of these 

 annual rings depends especially upon the fact that the growth 

 of the perennial stem does not take place continuously, but is in- 

 terrupted during certain periods of the season, for instance dur- 

 ing the winter or during the dry seasons of tropical climates. 

 And since the tissues which are formed at the beginning of eacli 



