124 



MINUTE STRUCTURE OF THE STEM. 



ring of liber may surrounrt tlie whole of the wooclj- portion, or 

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

 ments is common in the ^'ascular cr3-ptogams (see 354). 



373. The piUi of the stem consists of parenchj-ma frequentl}- 

 intermingled with other structural elements in small amount,-' 

 especiallj' long fibres, woody prosenchj'ma, 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) cr3-s- 

 tal-cells, in which er3-stals are 

 formed ; (3) inactive cells, 

 which, having lost the power 

 of receiving starch or other 

 products, remain empt}'. 



These apparentlj^ unimpor- 

 tant distinctions have been 

 shown by Gris ^ to be valuable 

 in the identification of consid- 

 erable groups of plants. Pith 

 composed of active or inactive 

 cells alone is termed bj^ him 

 homogeneous ; that wliich 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 closely 

 approximated (see also 207). 



375. The stem develoi)s from the bud by extension of its 

 internodes. When these have attained a certain length, different 



1 The peculiar structxirps fonnil occasionally in the periphery of the pith of 

 Sarnhncus, 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 uiid Westphalens, 1866, p. 1 ) . 



^ A detailed account of the orders of ])lants examined by Gris will be found 

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

 from the same is given in Ann. des ."^i'. nat., ser 5, tome xiv. , 1872, p. 34. 



Fia, 101. Clethra aliiifolia. Longitudinal section tlirougli tlie i-eticulated pitli of a 

 young brancli; eacli active cell contains a nucleus anil chlorophyll grains, December. 

 (Gris.) 



