J -8 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 abeady 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 



FtG. ^44^ — A. A third poftion of a vessel from the rhizome ofPtefis agutli'nd. The oblique scalarjfbrm end [/) 

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

 vessels. /? is the part ;<; {in -.^) more highly magnified ( x 375)- C and D very thin loQgitujlinal 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-tubesj 

 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. 



