160 



BOTANY 



PART I 



the roots or through the medullary rays to be stored in the living 

 cells of the wood. 



The different tissues of the bast are often arranged in very regular 

 tangential bands only interrupted by the medullary rays (Fig. 184). 

 The periodicity of the cambium is not, however, evident in the bast, and 

 there are no annual rings. The cambium continues to produce bast 

 after the formation of the autumn wood has ceased. 



In the Lime, for example (Fig. 184), there is an alternation of zones of sieve- 

 tubes (v) with companion cells (c), starch-containing bast parenchyma (p), cells 

 containing crystals (k), bast fibres (I), and flattened cells of bast parenchyma (p), 

 followed again by sieve -tubes. The differences in the appearance of the bast of 



cot 



FIG. 185. Transverse section of the outer part of a one-year-old twig of Pyrus cominunis made in 

 autumn. It shows the commencement of the formation of the periderm. p, Cork; pg, 

 phellogen ; pd, phelloderm ; col, collenchyma. The cork cells have their outer walls thickened 

 and have brown dead contents, (x 500. After SCHENCK.) 



different woody plants are due to the greater or less diameter of the sieve-tubes, 

 the presence or absence of bast fibres, and to the mode of arrangement of the 

 various elements. 



In the Pine and various other Abietineae, rows of cells with abundant 

 albuminous contents occur at the edges of the medullary rays (Fig. 176 em}. They 

 are in close contact with the sieve-tubes and connected with them by sieve-pits, 

 and become empty and compressed at the same time as the sieve-tubes. In 

 Dicotyledons the medullary rays in the bast are more simply constructed than in 

 the wood. The pitting of the cells of the medullary rays of Dicotyledons, which 

 connects them not only with the bast parenchyma but also with the companion 

 cells of the sieve-tubes, stands in relation to the taking up of assimilated material 

 as it is passing downwards. 



Effect of the Secondary Thickening on the Tissues external to 

 the Cambial Ring. 1. Dilatation. Since the cambium continues 



