486 



PHYSIOLOGY. 



times to become holes, by the absorption of the primary membrane which 

 formed a kind of diaphragm over the outer end. 



These pitted markings may be circular, oval, or elongated transversely 

 or more or less obliquely, so as to approach to the appearance of slits. 

 Sometimes the. later secondary growths do not extend quite to the edge 



Fig. 534. 



Fig. 532. 



Fig. 533. 



Fig. 535. 



Fig. 536. 



Fig. 532. Liber-cell of Periwinkle. Magn. 75 diam. 



Fi*. 5*i. Fragment of the cell in tig. G/,2, magnified 300 diam. (The spiral lines on the- urn ><>- 



Bite side of the cell show th-o'.'-h ami cross.) 

 Fig. 534. Vertical section of epidermis of Vucum album, with many th ickening layers. Magn. 



Fig. 535. Transverse section of cells of the petiole of Nymphaea alba, showing the larniiml<-rl 



WHil. j>iugii. 500 dijini. 

 Fig. 536. Fragment of a pitted duct of Laurun sassafras. Magn. 200 diam. 



of the aperture in the earlier layers, and the successive layers may so 

 retreat from this edge that the canal becomes at length fapnei-*hftped ; 

 in this case the pit, \vhon Been in front, presents a doubl" oufJj: . . one 

 corresponding to the outer end, the other to th<; inner and wider end 

 (fig. 530;. 



