46 



ON THE PLANT-CELL. 



* M 



I "' 





distance from the pore, when seen together with it from the plane sur- 

 face, is diminished (fig. 28. a, b) ; and, 2dly, the thickness of the air- 

 clefts between the cell-walls ; for 



28 j if these are very flat, the bor- 



dering surfaces will appear almost 

 parallel, and the edge is either 

 not at all or in a very slight 

 degree darkened, so that it cannot 

 be observed. If these cases are 

 placed in profile, as in fig. 28. 

 a, b, c, d, this explanation will be 

 understood. If we examine the pro- 

 cess of development in the large 

 and easily observed porous vessels of the cambium of the Willow, the 

 Lime, the Poplar, and the Maple, we shall find that all of them present 

 dark spots, which resemble those of the external circle of the air-clefts : 

 a clean transverse section of these spots is difficult to obtain ; but, from 

 our knowledge of optical phenomena, this dark spot may with certainty 

 be referred to the presence of a bubble of air. At this time no pore is 

 present : this is generally formed at a subsequent period. If these facts 

 are placed together, we may arrive at the conclusion that the air-cleft is 

 universally present previous to the appearance of the pore. The con- 

 sequence is, that, as the changes of matter by which the cell is nourished 

 can only take place during the contact of two cells, the cells are not 

 nourished at those points where the air exists between their walls : thus 

 the pore and its canal originate as a partial atrophy of the cell-wall. 

 Therefore, in all the transitions between porous cells through the netted 



29 



cells into the pure spiral, we must seek the cause of the division into 

 separate spirals not in the cell itself, but in its circumference. This 



Schulertia disticha, seen from the surface and in transverse section. The dotted lines 

 explain the relation which the two aspects bear to each other. 



28 Semidiagrammatic. Transverse sections of the pores, a, Pores small, in relation 

 to the spot where the neighbouring cell-walls separate from each other, b, Pores 

 large, in relation to this spot, c, The separation of the cell-walls so small that it only 

 appears as a black streak, d, The separations not observable between the cells are 

 apparently homogeneous layer rings, in which the pores terminate. 



29 p orou s cells from the perispeum of the ivory nut. 



!0 Transverse section of an intercellular passage, with the three portions of cell-wall 

 which forms it. The larger pores in the deposit-layer of the intercellular passages, as 

 well as the smaller ones on the double cell-walls, are seen. The corners of the inter- 

 cellular passages are rounded off by a peculiar deposit. 



