126 IX. THE DICOTYLEDONOUS STEM HERBACEOUS TYPES. 



branched, but not anastomosing, cells of which we have already 

 (p. 19) studied one of the contents in the form of peculiar starch 

 grains. These latex-cells are best studied in the stems of 

 one of the semi-succulent greenhouse Euphorbias, such as E. 

 splendens or E. jacquince flora. Tangential sections through the 

 thick cortex, especially if treated with iodine, show fairly regular 

 parenchymatous cells, and amongst them the latex-tubes, recog- 

 nisable from their moderately thick walls and contents. The 

 walls, as chlorzinc iodine will show, are of cellulose, smooth and 

 elastic ; they have a lining layer of cytoplasm, in which are 

 numerous nuclei ; otherwise are filled with a colourless, ap- 

 parently granular latex enclosing the starch grains referred to. 

 By careful maceration these branched unsegmented latex-tubes 

 can be extracted free, and to a considerable length. 



Bicollateral Bundles of Bryonia. Sieve-tubes. In almost all 

 the Cucurbitaceae, and certain other Dicotyledonous groups, we 

 find isolated vascular bundles, with two bast strings, one on the 

 outer, the other on the inner, side of the wood. These vascular 

 bundles are therefore constructed bicollaterally, The outer bast 

 is separated from the w r ood by a cambium of limited activity, the 

 inner bast impinges immediately upon the inner limits of the 

 wood. If the white Bryony of our hedgerows (Bryonia dioica), 

 the only British cucurbit, is available, it will serve admirably for 

 the study of the stem ; otherwise recourse must be had to on,e or 

 other of the cultivated varieties of Cucurbita Pepo, such as the 

 cucumber, or vegetable marrow. The stem-part used must not 

 be from anywhere near the growing point, or the large and very 

 striking ducts will not be fully formed, nor the sieve-tubes com- 

 pletely developed ; material had best be taken, therefore, from 

 the more mature, discoloured parts of the stem, and preserved in 

 alcohol. The vascular bundles, as examination of the cross- 

 section (which has been stained in aniline blue and mounted 

 in glycerine) with a low power, or even with the naked eye, will 

 show, are arranged in two concentric rings, usually five in each 

 ring. Those in the outer ring are small, and placed opposite the 

 angles of the stern ; those of the inner ring are much larger, and 

 alternate with the outer ones. The protection of the inner parts 

 of the stem is subserved by a pericyclic ring of sclerenchyma 

 fibres, the elements of which have stained far more deeply than 

 the large-celled ground-tissue. Externally to this ring, and 

 separated from the central cylinder, or stele, by a layer of cells, 



