LESS, xxxi GENERAL CHARACTERS 415 



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

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

 bundles ( V. B). (x2.) 



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

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

 (sp.v). (x6.) 



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

 stem, showing apical cell (ap. 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. sk), sieve-tubes (sv, t), 

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



D, a single parenchyma cell, showing nucleus (mi), 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. p/i), with intercellular spaces (i. c. sp), 

 a stoma (st) in the lower epidermis, and hairs (h). 



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

 (si) 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 (c, par ; D) considerably longer 

 than broad, their long axes being parallel w T ith 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. 



