54 



VEGETABLE HISTOLOGY. 



reagent, followed after a time by strong hydrochloric acid. The 

 cork cells remain unchanged, while the bast cells assume a fine 

 reddish-purple color. The chemical substance composing the 

 bast cells is known as lignin. 



The fibrous nature of the cells is seen only in longitudinal 

 section. 



6. The next layer is a not very broad zone of small-celled 

 tissue with blue stained walls. Under high power this is com- 

 posed of two sub-layers; the outer one consists of larger cells, 

 more rounded, of unequal size, irregularly arranged and made 

 up of two kinds of thin-walled cells, sieve tissue and a variety 

 of parenchyma. These constitute the so-called soft bast. 



1. The inner layer of the two sub-layers is composed of very 

 small cells, rich in protoplasm, very closely packed, in radial 

 rows. These are primary meristem 

 cells and form the so-called cam- 

 bium zone of the stem. It is in this 

 zone where growth takes place most 

 actively, by which the stem con- 

 tinues to increase in diameter. 

 When the bark is stripped from a 

 stem the rupture takes place in this 

 succulent fragile zone. 



8. Next the cambium is a layer 

 more or less broad, according to the 

 age of the stem, composed of cells 

 for the most part like the bast fibres 

 (see 5) and stained brown. These 

 are wood or libriform fibres (ligni- 

 nified) . Scattered among them are 

 other cells of larger diameter whose 

 walls are also thickened and stain- 

 ed brow T n. These constitute the vasi- 

 form or vascular tissue, composed 

 of ducts and tracheids of vari- 

 ous kinds. Their function is to 

 strengthen and also to convey nu- 

 triment. All these cell-walls are 

 lignified and behave like the bast 

 fibres towards phloroglucin stain. 



9. Interior to the wood circle is 

 the pith, composed of large-celled 

 parenchyma, with large inter-cellu- 

 lar spaces and starchy contents. 

 The cells on the exterior are 



smaller than those towards the 

 center and more compactly arrang- 



ed. Many Of the cells Of the pith 



and the outer parenchyma layer 



d 



f 



Fig. 35. Geranium stem, cross-sec- 



chyma ; c, parenchyma of middle 



* ark c ^ a m st r n e e s . ; V^VTo* 



mixed wood fibres and vessels; 



'*' 



