FORMATION OP CELLS— TISSUES 



317 



entire structure of many of the lower plants, such as the Algis, 

 Fungi, and most Mosses, wliiola are hence frequently termed 

 Cellular Plants. 



Peosenchyma. — Besides these collections of isodiametric 

 cells we find others wliose lengtli greatly exceeds tlieir other 

 diameters. They are usually pointed in shape, their ends over- 

 lapping, or extending in between the ends of their immediate 

 neighbours. These collections constitute what is known as 

 prosencTiyvia. As in the case of parenchyma the walls of the 

 cells, or fib ITU, are sometimes lignified, pitted, &c. Prosenchyma 

 is largely present in woody tissues ; it is not, however, confined 

 to them, but may be found as isolated strands occupying various 

 positions in the plant body. In some forms the fibres are 

 branched {fig- 680) . The cells of coUenchyma and sclerenchyma 

 are frequently fibrous. 



The secondary wood of the Coniferse (fig. 638) is entirely 

 composed of prosenchymatous tissue, the constituent cells of 

 which, fibrous in fornr, are known as trache'ids. They are 

 marked by the peculiar bordered pits ah-eady described. These 

 may.be regarded as intermediate between prosenchyma and the 

 vascular tissue described below. 



Vasoulak Tissue. — A further modification of the arrange- 

 ment of cells is found in plants, consti- 

 tuting what is known as vascular tissue. 

 The cells of this tissue are frequently 

 fused together, the end walls between them 

 becoming absorbed to a, greater or less 

 extent. A row of cells whose separating 

 walls have thus disappeared is known as 

 a vessel. Vascular tissue includes two 

 forms, tracheal tissue and sieve tissue. 



Tracheal Tissue. — This is usually 

 composed of lignified cellswhich are placed 

 end to end, forming columns, and which 

 havelosttheirprotoplasm. Theyare pitted 

 in the various manners already described, 

 forming spiral, annular, scalariform, or 

 reticulated vessels {fig. 682). When the 

 end walls are completely absorbed they 

 form true vessels or trachece. The 

 absorption is not always complete ; 

 indeed, in some cases columns of such 

 thickened cells are found where the end walls persist, and are 



Fig. 683. 



Fig. 682. 





GzS 



/'7(/. 682. Beadeil pitted ves- 

 sel. Fl,/. HSi. Pitted 



traclicid tpiTLiinatiug ob- 

 liquely, and ' showing 

 that the partition wall 

 has been incompletely 

 absorbed. 



