APIUM GRAVEOLENS 477 



the end walls are characteristically thickened. (Fig. 144, A-C.^ 

 These thickenings are lenticular, occurring only on the portion of 

 the wall which is later removed during the maturation of the ves- 

 sel, while the remainder of the end wall is thin. After the end 

 wall has thickened, the vessel segments develop secondary wall 

 thickenings of the spiral, scalariform, or reticulate types which 

 are deposited on the longitudinal walls and also on the thin 

 portion of the end walls. The disintegration of the end wall of 

 the vessel segment occurs when the longitudinal secondary wall is 

 mature and the protoplast is beginning to disappear. (Fig. 144, 

 F, G.) At this time, the lenticular portion of the end wall becomes 

 thinner until finally it completely disappears, indicating that the 

 process is one of dissolution. 



Ontogeny of the Phloem. — The protophloem consists of 

 sieve tubes and companion cells, each tube having one of the 

 latter. (Fig. 2.45.) As development proceeds, some of the first- 

 formed protophloem is obliterated by crushing and additional 

 sieve tubes are formed centripetal to them. The metaphloem has 

 less parenchymatous tissue; and the sieve tubes, though similar 

 in structure to those of the protophloem, are larger. Each tube 

 usually has one companion cell, but sometimes there are two short 

 ones. (Fig. 145, G.) 



In the formation of the protophloem, the procambial mother 

 cell divides several times. The next to final division forms a 

 parenchymatous cell and the mother cell, after which a longitudinal 

 division of the latter produces a sieve tube and companion cell. 

 In the development of the metaphloem, some of the parenchy- 

 matous cells may develop directly from procambial cells; but, in 

 most instances, a procambial cell divides longitudinally. One 

 daughter cell becomes a parenchymatous cell and the other, the 

 mother cell of a sieve tube and companion cell. In the division of 

 the mother cell, the larger of the daughter cells forms the sieve tube. 



In maturation, the sieve tube first develops thickenings on the 

 longitudinal walls, and the sieve plate thickenings occur later 

 at about the time that the nucleus disintegrates. The wall thicken- 

 ing appears to be uniform on all longitudinal walls, but the one 

 adjacent to the companion cell may or may not be thickened. In 

 old age, prior to obliteration, the walls of the sieve tube lose 

 their thickening, and this is followed by crushing as a result of 

 the growth and enlargement of adjacent parenchymatous cells. 



