SUNFLOWER STEM 39 



with that of an old one. The conclusion will naturally be drawn 

 that the stem increases in girth as it grows older, and since the 

 outer tissues neither peel off, nor do the individual cells increase 

 greatly in width, longitudinal radial divisions of the cells are 

 the only alternative. 



Before leaving the cortical tissue it must be noticed that the 

 bundle-sheath or endodermis, which is the inmost layer of the 

 cortical tissue, and which is easy of observation in the younger 

 stem, may be identified in these sections, though with difficulty 

 (see below, p. 45). It lies immediately abutting upon the thick- 

 walled sclerenchyma, but it is often difficult to distinguish it 

 in old stems. 



Within this are 



4. The vascular bundles. Select one of the largest of these 

 for more minute examination : it will be found to consist of 

 two well marked masses of thick- walled tissue, peripheral and 

 central as regards the stem, with a transparent thin-walled 

 portion between them. Further, on examining the latter more 

 carefully, it will be seen that the external part of it has thicker 

 walls, and is less regularly arranged than the central portion, 

 and must accordingly be distinguished from it. We have 

 thus four portions of the bundle which, taking them in 

 succession from the periphery to the centre, are named as 

 follows : 



(i.) Sclerenchyma. 

 (ii.) Soft Bast or Phloem. 

 (iii.) Cambium. 

 (iv.) Xylem. 



i. Examine first the sclerenchyma. This appears as a half- 

 moon-shaped mass of tissue consisting of elements with 

 rounded cavity, in which may be recognized the remnants of 

 protoplasmic contents. The walls are thick, and lignified 

 (yellow with acidulated aniline sulphate, or with chlor-zinc- 

 iodine, see p. 25). They also show differentiation into layers, 

 of which the most prominent is the bright-looking middle 

 lamella. Perpendicular to the internal surface of the walls 

 pits may be seen. 



