THE TISSUES OF PLANTS. 25 
regular in their form, and with tapering extremities (Fig. 
19). Their walls are but slightly thickened, and are 
marked with spirals and pits. When the wall between two 
contiguous cells breaks through or becomes absorbed, the 
2 
& 
© 
° 
a 
OCS 
© caVlo©oe® 
o 
@ 
| rc) 
OO! @ 6 © 
Fie. 18. 
Fig. 18,—Ends of several tracheids from the wood of a Pine, showing bordered 
pits Magnified 325 times. 
Fie, 19.—Tracheids from the stem of Laburnum. m,™m, cells of a medullary 
ray. Atg,apartition is broken through. Magnified 375 times. 
close relation of such tracheids to spiral vessels is readily 
seen. 
Tracheids may be regarded as composing a less differen- 
tiated form of tissue, related on the one hand to true tra- 
cheary tissue and on the other to fibrous tissue, 
