,38 



LECTURE VIII. 



mostly thin walls, and, at least in the younger organs, are filled with slimy proteid 

 matter. The transverse septa, which lie closer to one another the older the part of 

 the plant had already become before the formation of the sieve-tubes, are also 

 of a soft, or even gelatinous consistence ; and they are never entirely, or to any 

 great extent absorbed; but by absorption taking place at isolated spots, they 

 become transformed into a network of ridges, in the meshes of which lie the canals 



Fig. 144.— y*. A third portion of a vessel from the rhizome of Pteris aqiiitina. The oblique scalariform end (/) 

 and a portion of the lateral wall in surface view: the areas not pitted correspond to the angles of neighbouring 

 vessels. B is the part x (in A) more highly magnified ( x 375). C and D very thin longitudinal sections perpendicular 

 to the lateral wall of two neighbouring vessels; showing the thin primary wall, on which are situated the sections of 

 the thick projections between the pits. (After De Bary.) 



by means of which the segments of a sieve-tube, situated one over the other, com- 

 municate, and through which the slimy contents can be pressed. Where sieve-tubes 

 border immediately on one another laterally, so-called sieve-plates may also be 

 formed on their side-walls, the structure of which resembles that of the sieve-like 

 transverse septa. 



Amongst the tracheal structures of the xylem, and still more between the 

 sieve-tubes, is found more or less abundant, parenchymatous, mostly thin-walled 

 tissue : this consists of more or less elongated, soft cells, which contain fluids 

 of various kinds, and frequently starch. 



