THE YECtETABLE CELL 



289 



while it is still growing in surface, the wall becomes thickened 

 by the deposition of successive layers over those already formed. 

 A transverse section of many cell-walls shows traces of this 

 mode of thickening, the successive layers appearing as shells of 



I 



Fig. 627. Fia. 628. 



Fie. 629. 



FiGf. 630. 



Fig. 631. 



Fig. 627. Spiral cell. Fig. 628. Anuular cell. Fig. 629. Hetioulaterl 



cells. Fig. 630. Pitted and reticulated cell. Fig. 631. Wood-cells of 



the Yew (_Ta.-cif.i baccafa). After Von Mnlil. 



substance lying one upon another. Such a cell-wall is said to be 

 stratified (Jigs. 624 and 625). Viewed longitudinally, the walls 

 often appear covered with delicate oblique striations which may 

 run regularly in one direction only, or may be crossed by others. 



Fig. 632. 



Fig. 633. 



Fig. 634. 



Fig. 635. 



Fig. 636. 





wesi 



CB3 



Fiy.G'62. Simple spiral vessels. Fig.G^S. Annular vessel. Fig.GS'i. Re- 

 ticulated vessel. i^i(7. 635. Cylindrical seal ariformvesselsof the Vice. 



Fig.Qdii. Pitted tracheid terminating obliquely, and showing thattlie parti- 

 tion wall has been incompletely absorbed. 



This is due to the way in which the protoplasm deposits the new 

 particles of cellulose upon the layers aheady existing, a succes- 

 sion of spiral bands being laid down with their edges in contact. 

 Such striations can be well observed in many of the elongated 

 VOL. I. u 



