CHAPTER II. — PLANT TISSUES. l6l 



markings, in this respect resembling ducts, but they differ from 

 the latter in the fact that they do not become confluent end to 

 end into tubes. Where they occur in association with wood 

 cells, as is most commonly the case, they are usually larger than 

 these in transverse diameter, and have less tapering, merely 

 oblique or even square ends, though there are some exceptions 

 to these rules. When arranged end to end in linear series, they 

 are indistinguishable from ducts, save by the imperforate trans- 

 verse or oblique partitions. 



In the Pines and other Gymnospermous plants a peculiar kind 

 of tracheid takes the place of wood cells, and also for the most 

 part, of ducts, giving to the wood of these plants peculiarities 

 which enable us readily to distinguish it from that of other plants. 

 If a radial section of the wood of the common White Pine be 

 made, the elongated, fusiform cells of the woody zone will be 

 seen to possess numerous rounded pits, each of which, in this 

 view, looks like two circles, one within the other. They occur 

 mostly on the radial faces of the cells, as is shown by comparing 

 the radial, transverse and tangential sections. The comparison 

 will also show us that the bordered pits, as they are called, are 

 lenticular areas in the common wall between two adjacent cells. 

 These areas have their lateral walls perforated centrally with a 

 circular or oblong perforation. It is this which, in the radial 

 view, gives rise to the optical impression of an inner circle in 

 each pit. The perforation, however, does not extend, except in 

 very old wood, completely through the common wall from one 

 cell to the other, but there still remains stretched across the 

 cavity of each area a delicate separating membrane. The 

 structure will be understood by reference to Figs. 414, 415 and 

 416, which represent small portions, respectively, of transverse, 

 radial and tangential sections of White Pine wood. 



The parenchyma cells, with bordered pits, which occur 

 immediately within the bundle sheath in the leaves of Pines, may 

 be regarded as transition forms between ordinary parenchyma 

 and the tracheids just described. Indeed, by some they are 

 classed as tracheids. They have already been shown in Fig. 399. 



Just as wood cells graduate into ducts, they also, on the 

 other hand, graduate toward parenchyma, and we frequently 

 find among the woody tissues rows of rather thin-walled 



