124 MINUTE STKUCTUKE OF THE STEM. 



ring of liber ma}- surround the whole of the woody portion, or 

 the wood may surround the liber. The former of these arrange- 

 ments is common in the vascular cryptogams (see 354). 



373. The pith of the stem consists of parenchyma frequently 

 intermingled with other structural elements in small amount, 1 

 especially long fibres, woody prosenchyma, and latex-cells. 



The parenchyma cells of pith have been classified in the 

 following manner: (1) active cells, having the office of storing 



starch and other assimilated 

 products for a time ; (2) crys- 

 tal-cells, in which ciystals are 

 formed ; (3) inactive cells, 

 which, having lost the power 

 of receiving starch or other 

 products, remain empty. 



These apparently unimpor- 

 tant distinctions have been 

 shown bj* Gris 2 to be valuable 

 in the identification of consid- 

 erable groups of plants. Pith 

 composed of active or inactive 

 cells alone is termed by him 

 homogeneous ; that which con- 

 tains more or less of both kinds of cells, heterogeneous. The 

 arrangement of the elements in heterogeneous pith is so nearly 

 constant as to have much interest for the systematist. 



374. The medullary rays comprise the conjunctive parenchy- 

 ma, which lies between the bundles in the stems of normal 

 dicotyledons. The cells are for the most part much flattened 

 radially, always so in those cases where the bundles are closety 

 approximated (see also 207). 



375. The stem develops from the bud by extension of its 

 internodes. When these have attained a certain length, different 



1 The peculiar structures found occasionally in the periphery of the pith of 

 Sambucus, and sometimes in the bark, have been mistaken for fungi, but have 

 been shown by Oudemans and by Dippel to be receptacles for a very heteroge- 

 neous mixture of tannin and other matters ( Verh. d. Nat. Vereins d. Preussens, 

 Rheinlande und Westphalens, 1866, p. 1). 



2 A detailed account of the orders of plants examined by Gris will be found 

 in Nouvelles archives du Museum, t. vi. fasc. 3, 4, p. 201 (9 plates). An extract 

 from the same is given in Ann. des So. nat., se> 5, tome xiv., 1872, p. 34. 



PIG. 101. Clethra alnifolia longitudinal section through the retlculatd pith of a 

 young brand. ; each active <-ell .-onmim. a, nucleus and chlorophyll grains, December. 

 (Gris.) 



