. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 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 fo
. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 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,
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