THE TISSUES OF PLANTS. 



25 



regular in tteir 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 



Fio. 18. Fis. 19. 



Fig. 18.— Ends of several tracheids from the wood of a Pine, showing bordered 

 pits Magnified 3.J5 times. 



Fig. 19. — Trachei'ds from the stem of Laburnum, m, m, cells of a medullary 

 v&j. At gr, a partition 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. 



