FOEMATION OF CELLS— TISSUES 



321 



and circular, being broad and extending quite across the side of 

 the cell. 



The annular, reticulaiecl , and soalariform vessels liave 

 commonly tapering points like the spiral vessels, and thus over- 

 lap at their extremities when they come in contact (fig. 693). 

 But in other instances they terminate more or less obliquely, or 

 by flattened ends, like most pitted vessels. 



Sieve tissue. — This is generally associated with some forir 

 of tracheal tissue in certain strands of tissue known as vascular 

 bundles. It occasionally exists as isolated masses in the pith 

 or cortex of steins. In young roots it is found in independent 



Fia. 695. 



Fig. 695. Sieve tubes aud companion cells from the stem of Cucurh 

 transverse ami longitudinal section, s.t. Sieve tube. s.p. Sieve plate. 

 t: Companion cells, x 540. After Strasbnrger. 



strands, side by side with similar strands of tracheal tissue. 

 The cells of which it is composed are thin-waUed and never 

 lignified. Two kinds of cell are found in it in the most highly 

 developed forms, the sieve tube proper and its companion cells. 

 The sieve tube is composed of a number of cells or segments 

 placed end to end (fig. 695). The separating wall is seldom much 

 thickened, but has deposited upon it on both sides a peculiar 

 material known as callus, probably a modification of the original 

 material of the wall. The separating wall generally forms a 

 sieve plate, being perforated by a number of apertures, which are 

 lined by the callus deposit, and through which the contents of 



VOL. I- Y 



