352 



MANUAL OP BOTANY 



described above the development of wood starts by the differentia- 

 tion of a spiral or annular vessel or row of tracheids situated, 

 with few exceptions, at the apex of the wedge, forming the pro- 

 toxylem (fig. 738, px). This is the only point at which spiral or 

 annular vessels ever occur in such a bundle. The differentiation 

 then proceeds gradually backwards or oentrifugally, forming the 

 prima/ry wood, which consists of vascular and parenchymatous 

 elements with thickened lignified walls. The vessels and 



Fifi, 738. Collateral bundle of Ricinus. en 

 c. Cambium, px, Protoxylem. x. Xylem. 



Eudodermis. ph. Phloem, 

 if.c. Interfascicular cambium. 



tracheids are variously pitted, as already described, and they 

 lose their protoplasmic contents. 



The outer part or bast of this type of bundle begins to be 

 differentiated at a place at the exterioi of the wedge-shaped 

 mass, where a few sieve tubes and their companion cells may 

 be observed. These constitute the protophloem. Their shape 

 becomes altered by pressure within the bast ; their walls appear 

 swollen and their cavities almost indistinguishable. The 

 differentiation of the rest of the bast proceeds then oentripetally 

 towards the wood. In some cases the wood and bast come to 

 meet as above described ; in others a band of the procambium 

 between them does not become permanent tissue, but retains its 



