658 



SECONDARY GROWTH IN THICKNESS 



their walls. The two sets of cells are connected by means of numerous 

 pits, through which an active interchange of materials is maintained ; the 

 companion-cells likewise enter into close relation with the medullary rays. 

 Among the Abietineae, and in some Cupressineae and Taxodineae, the 

 phloem-rays contain in addition to starch-storing or carbohydrate- 

 conducting elements, the so-called " albuminous cells " discovered by 

 Strasburger ; these cells, which generally occupy the upper and lower 



me 



ml' c^ 



ine 



ml 



ml 





sr 



P 



Fig. 273. 



Secondary medullary ray of Pinus Pumilio (at the beginning of July), me, albu- 

 minous cells ; mt, tracheidal cells; ml-ml', row of conducting parenchyma cells; sr, 

 sieve-tubes; c, cambium ; ft, fibrous tracheides. 



margins of the phloem-rays, differ from the starch-containing elements 

 in their greater width and often also in the large size of their nuclei 

 (Fig. 273, m,e). They adjoin the secondary sieve-tubes and communi- 

 cate with them in the Abietineae, and probably also in the rest of the 

 above-mentioned Conifers by means of sieve-pores. They lose their 

 contents at the same time as the sieve-tubes with which they are 

 associated. 



In the course of secondary growth in thickness, the circumference 

 of the extra-cambial region naturally increases to a very considerable 

 extent, since it is continually being pushed outwards. The (passive) 

 tangential distension upon which this increase depends, is, of course, 

 restricted to the living constituents and cannot, for example, take place 

 in the fully developed periderm or bark. Most trees shed the outer- 



