152 



BOTANY 



PART I 



The medullary rays in the wood of Gymnosperms are numerous, 

 and for the most part only one layer of cells broad (Figs. 175 m, 

 177 sm, tm; 179 ms). Every tracheide abuts in the course of its 

 length upon one or more of these medullary rays. The cells of the 

 medullary ray are elongated in the radial direction ; they contain 

 abundant starch and are associated with intercellular spaces (Fig. 

 177 i). They serve to transfer the products of assimilation, formed 

 in the leaves and conducted downwards in the bast, in a radial 

 direction into the wood of the stem or root, where storage takes 



sm 



FIG. 176. Radial section of a Pine stem, at the junction of the wood and bast, s, Autumn 

 tracheides ; t, bordered pits ; c, cambium ; v, sieve-tubes ; vt, sieve-pits ; t tm, tracheidal 

 medullary ray cells ; sm, medullary ray cells in the wood, containing starch ; sm', the same, in 

 the bast; em, medullary ray cells, with albuminous content, (x 240. After SCHENCK.) 



place ; they also conduct water from the wood outwards. The 

 medullary rays are suited to perform these functions, since, as has 

 been seen, they extend into both the wood and the bast (Figs. 175 B, 

 176, 179). The intercellular spaces communicate with the intercellular 

 system of the cortex and allow of the necessary gaseous exchanges 

 between the living cells in the wood and the external atmosphere. 



In certain Gyranosperms, especially the Pines, single rows of cells of the 

 medullary ray in the wood (usually the marginal rows) are tracheidal and without 

 living contents ; they are connected with one another and with the tracheides by 

 means of bordered pits (Fig. 176 tm). They are protected against compression by 

 the living turgescent cells of the medullary ray by means of special thickening of 



