200 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[35. 



Juglans, Sambucus, Rhamnns, Ulmus, Populus); or there may be 

 scattered groups of fibres (e.g. Cinchona, Morus, Larix, Celtis, 

 Ficus elastica). 



The secondary bast does not, as a rule, attain so considerable a 

 size as the secondary wood, nor does it exhibit annual rings : this 

 is due to the fact that, except in some fleshy roots, it is formed in 

 smaller quantity, and further, to the fact that the older bast be- 

 comes crushed and flattened by the development of the more 

 internal layers subsequently formed. 



x ft 



moMigJ/grCTiP 



FiG. 151. "Radial longitudinal section of the wood of the stem of a Pine, along- the length 

 of a medullary ray q p q, consisting of six horizontal rows of cells, one above the other : 

 t tracheids with bordered pits ; the tracheids h with smaller bordered pits are the autumn- 

 wood of one year, those to the right with larger pits constitute the spring- wood of the next 

 year; q tracheidal elements of the medullary ray ; p true cells of the ray : where the cells 

 of the medullary rays abut on the tracheids the pits are simple and large ( x 300). 



The structure of the secondary conjunctive tissue (medullary rays). 

 The cambium-ring not only adds to the existing primary medul- 

 lary rays, but gives rise to new (secondary, tertiary) rays in the 

 successive years of growth (see Fig. 149), amongst the vascular 

 tissue. 



The cells of the medullary rays are typically parenchymatous, 

 somewhat brick-shaped, with their long axes along radii from the 



