FORM OF THE PLANT-CELL. 



45 



23 



x* 



Xxx 

 XXX 



24 



5^ 



cell (fig. 25.). In the woody tissue of the Coniferce, in the porous 

 vessels of the wood, in almost all vessels with oblong pores, the pore 

 may be seen surrounded by a double circle, an inner one easily recog- 

 nised as the pit in the deposit-layer, and an outer wider circle (fig. 26.). 



26 



Sometimes three circles are seen. If we compare the transverse section 



of these pits with their appearance on a flat surface, we shall see how 



this takes place (fig. 27.). In the region of the 



canal of the pore, the cell-walls, which at first 



lay upon one another, become separated, and 



leave a lenticular space between them, which is 



filled with air. The edges of this space appear 



from the surface as an external circle. The one 



or two inner circles are produced by the canal, as 



seen in fig. 27. This appearance of the external 



circle admits of two explanations: 1st, its greater 



83 Porous cells from the parenchyma of the stem of Arundo Donax. 



24 Porous cells, from the petiole of Hoya carnosa. 



25 Porous cells of the petiole of Cycas revoluta. Small pores are found where 

 the cells are xinited to each other, but large ones where the cells open into the inter- 

 cellular passages. 



>6 Porous cells from the wood of Abies excelsa. The pores are surrounded by a large 

 external circle. 



27 Semidiagrammatic. A single perfectly developed pore from the wood-cells ot 



