THE ANGIOSPERMAE : STEMS 



859 



after they are fully differentiated. This may lead to the rupture of their 

 end walls, so that open connection between tracheids is established and a 

 " vessel " is formed. Protoxylem may therefore consist of either tracheids 

 or vessels, but the difference in this case is accidental and irregular. Stretch- 

 ing of the protoxylem may break down the longitudinal walls as well as the 

 transverse walls, so that the whole element collapses. ' Nothing then remains 

 of the protoxylem but an intercellular cavity containing disorganized rings 

 or spirals of lignin. This is often observable in mature stems. 



Very different is the course of events in the formation of the mature type 

 of vessel in the metaxylem. Here the individual cell elements may be either 



t Protoxylem 

 Metaxylem 



Cambium m 



Sclerenchyma 



Pith 



Fig. 844. — Helianthus rigidiis. Longitudinal section 

 through a primary vascular bundle of the stem 

 showing various types of vessel thickening. 



long or short, but in either case they expand transversely at a ver\' early 

 stage, even while still in contact with the cambium, until they are consider- 

 ably broader, sometimes several times as broad, as the neighbouring tracheids. 

 Their transverse walls are dissolved, wholly or in part, before thickening 

 takes place, so that an open tube results, in which lies a file of separate 

 protoplasts. A visible cytoplasmic pattern then develops, which determines 

 the pattern of lignification, and this is continuous through many protoplasts, 

 which shows that although separate they are subject to some common 

 influence. Transverse expansion may continue during lignification, and 

 the pits may then become extended like transverse slits in the completed 

 woody wall. If these slits form a regular longitudinal succession the element 

 is called scalariform, but such elements are uncommon in Angiosperms as 

 compared with Pteridophytes. 



