LESS, xxix GENERAL CHARACTERS 347 



A, Transverse section of the stem of Pteris aqnilina, showing hypo- 

 dermis (hyp], ground parenchyma (par], sclerenchyma (set), and vascular 

 bundles ( V. B). (x 2.) 



B, transverse section of a vascular bundle, showing bundle-sheath 

 (b. sh\ sieve-tubes (sv. f), scalariform vessels (sc. v), and spiral vessels 

 (sp. v}. (x 6.) 



C, semi-diagrammatic vertical section of the growing point of the 

 stem, showing apical cell (op. c), segmental cells (seg. c), and apical 

 meristem (ap. mer) passing into permanent tissue consisting of epidermis 

 (ep\ hypodermis (hyp], ground parenchyma (par], sclerenchyma (scl), 

 and vascular bundles in which the sheath (b. s/i), sieve-tubes (sv. /), 

 scalariform vessels (sc. v), and spiral vessels (sp. v) are indicated. 



D, a single parenchyma cell, showing nucleus (), and vacuole 

 (vac]. 



E, cell of hypodermis. 



F, portion of a sieve-tube, showing sieve-plates (sv. pi). 



G, portion of a spiral vessel with the spiral fibre partly unrolled at the 

 lower end. 



H, fibre-like cell of sclerenchyma. 



I, portion of a scalariform vessel, part of the wall being supposed to 

 be removed. 



K, vertical section of a leaf of Pteris, showing upper and lower epi- 

 dermis (ep\ mesophyll cells (ms. ph), with intercellular spaces (i. c. sp), 

 a stoma (sf) in the lower epidermis, and hairs (A). 



L, surface view of epidermis of leaf of Aspidium, showing two stomata 

 (sf) with their guard-cells (gd. c). 



M, vertical section of the end of a root, showing apical cell (ap. c}, 

 segmental cells (seg. c), and root-cap (r. cp} with its youngest cap-cells 

 marked cp. c. 



(A, B, and D-K after Howes ; M from Sachs, slightly altered.) 



tissue elements are separated from one another, and can be 

 readily examined under a" high power. 



By combining these two methods of sectioning and 

 dissociation, the parenchyma is found to consist of an 

 aggregate of polyhedral cells (D) considerably longer than 

 broad, their long axes being parallel with that of the stem 

 itself. The cells are to be considered as right cylinders 

 which have been converted into polyhedra by mutual pres- 

 sure. They have the usual structure, and their protoplasm is 

 frequently loaded with large starch-grains. They do not fit 

 quite closely together, but spaces are left between them, 

 especially at the angles, called intercellular spaces. 



