24] 



CHAPTER I. THE CELL. 



105 



The Structure of the Cell-wall. When the cell- wall is at all 

 thickened it presents indications of structure. 



It presents, in the first place, a 

 layered appearance when examined in 

 longitudinal or transverse action (Fig. 

 60). This layering or stratification of 00 



the cell- wall is readily intelligible when 



FIG. 59. A cell with 

 pitted walls, from the 

 wood of the Elder (Sam- 

 bucus). A longitudinal 

 section showing the pits 

 in the lateral walls as 

 channels, a ; and in the 

 farther wall as roundish 

 spots, b. ( x 240.) 



FIG. 60. Transverse sec- 

 tion of a bast-cell from the 

 root of Dahlia variabilis (x 

 800) ; I the cell-cavity. K Pit- 

 canals which penetrate the 

 stratification ; sp a crack by 

 which an inner system of 

 layers has become separa- 

 ted. (After Sachs.) 



FIG. 61. Cells from the endo- 

 sperm of Ornithogalum unibella- 

 tum showing simple pits : 

 m pits seen in surface view ; p 

 closing membrane seen in lon- 

 gitudinal section ; n nucleus. 

 (x210: after Strasburger.) 



it is remembered that the thickening of the wall is due to the 

 deposition of successive layers from within. 



It presents, secondly, a delicate 

 striation, when examined in sur- -n 



face- view, the lines running at a 



\ 



FIG. 62. Oval bordered pits in the 

 wall of a vessel of Helianthus. A In 

 longitudinal section. B As seen from 

 the surface; t the pit; Tithe pit-chamber 

 (x 600). 



FIG. 63. Circular bordered pits on 

 the wood-cells of the Pine. A Seen from 

 the surface. B In section ; s the closing 

 membrane; m the middle lamella. CAn 

 earlier stage, in section. ( x 500, diagram.) 



