CELLS AND TISSUES 



19 



segments forming the vertical series enlarge in diameter without 

 much increase in length. Until the full size of the vessel is at- 

 tained, the segments are separated by definite end walls which may 

 lay down some secondary wall thickening. (Fig. lo.) When 

 the segments have attained full size and shape, the dissolution of 

 the end wall begins; and, in the transverse-porous type, the entire 

 end wall disappears, so that a continuous tube results. In Apium, 

 Esau (i6) found that the ontogeny of the vessel agrees in general 



Fig. io. A-C, successive stages in the ontogeny of the vessel in Cucurbita showing the 

 disintegration of the end walls of the vessel segments. 



detail with that given above, except that no deposition of second- 

 ary wall material occurs on the end walls. The wall thickening 

 appears to be primary and of a peculiar lenticular type somewhat 

 similar to the torus formed in some bordered pits. (See Fig. 2.44.) 

 Priestley and his coworkers (31) have described the perforation 

 of the end walls of several species of woody angiosperms as occur- 

 ring early in the ontogeny of the vessel segment and prior to the 

 initiation of secondary wall thickening; but note that "pectin 

 films are often left stretched across the perforated ends of the vessel 

 segments." They point out that because of these films there 

 is no sudden intermingling of the contents of adjacent vessel seg- 

 ments. Scott (38) has described a coenocytic development of 



