Elementary Botany 



surfaces. At first these layers are complete and continuous 

 like the original coat, but soon they are developed but slightly 

 or not at all at certain points, thus causing perforations in their 

 substance. As the holes thus produced in the contiguous 



Fig. 23.— I. Disc-shaped cell ; a uni- 

 cellular Alga, Coscinodisciis. II. 

 Crescent-shaped cell of a stoma 

 (guard -cell). 



Fig. 24. — Tetrahedral cells ; spore 

 of a Fern in various positions. 



layers correspond with one another, canals are gradually formed 

 leading towards the centre of the cell cavity. 



Such cells, when seen under a microscope, transmit the 

 light differently through their pore canals and through the 

 thickened walls, and hence they present a pitted or dotted 



Fig. 25. — Branched bast- 

 cell of the Larch. 



Fig. 26. — Moderately thickened pit- 

 ted parenchyma from the pith of the 

 Beech. 



appearance as though pierced by a number of holes (fig. 26). 

 These cells are known as pitted or dotted cells. 



In other cases the secondary deposit takes the form of a 

 spiral band round the wall, thus producing a spiral cell 



