MURi'ilULULiV UF lilGHER PLANTS. 



207 



etc.) the presence of a cambium is found only in the Dicotyledons 

 and extremely seldom in the Monocotyledons. The central 

 portion of the stele is frequently differentiated into a pith 

 of parenchymatic structure, the cells of which often contain large 

 quantities of starch (Figs. 220, 223). In addition in the pith, 

 Vv'e often find the same types of secreting ducts or cells as occur 



Fig. 116. Section of a four-year-old stem of a pine cut in winter; q, view in irans- 

 verse section; 1, radial-longitudinal section; t, tangential-longitudinal section'; f, spring 

 wood; s, fall wood; m, pith; i, 2, 3, 4, successive years' rings of growth in which ' shows 

 the dividing line; ms, medullary rays in transverse section; ms^, ms^^ medullary rays 

 in radial-longitudinal section; ms^^^, medullary rays in tangential-longitudinal sectior"; 

 c, cambium; b, bast; h, resin-canals; br, bork. After Strasburger. 



in the cortex (as in Apocynum), The pith may constitute a 

 homogeneous tissue or be broken, as in Phytolacca (Fig. 139, A), 

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

 divided into a row of broad cavities that are separated by thin 

 transverse walls of parenchyma. 



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 



