76 



BOTANY 



differences, there can frequently be distinguished three distinct layers 

 — a primary, a secondary, and a tertiary thickening layer. These 

 layers are deposited on the primary cell wall, which, in the case of 

 cells arising from cell division, is represented by the newly-formed 

 partition wall. The secondary thickening layer is usually the most 

 strongly developed, and forms the chief part of the cell wall. The 

 tertiary or inner layer is thinner and more highly refractive. In 

 special cases, but only in the formation of reproductive cells, an inner 

 thickening layer, completely detached from the others, is produced, 

 as in the formation of pollen grains and spores, which, enclosed only 



Pig. 83. — From the wood of the Pine, Pintis sylvestris. 

 A s Bordered pit, in surface view ; B, bordered pit in 

 tangential section ; t, torus ; C, transverse section of 

 a tracheid ; m, middle lamella, with gusset, m* ; 

 f, inner peripheral layer. ( x 540.) 



Fig. 84. — Cells from the endo- 

 sperm of Ornithogahim um- 

 iellatum. m, Pits in surface 

 view ; p, closing membrane ; 

 n, nucleus, (x 240.) 



by this inner membrane, finally become freed from the older thickening 

 layer. This process is often alluded to as Rejuvenescence ; in such 

 cases, it should be noted, there are, in reality, no new cells formed. 



The thickening of the cell wall seldom takes place uniformly over 

 the whole surface ; but some portions are thickened, while, at other 

 points, the original or primary cell wall remains unchanged. In this 

 way pores are formed which penetrate the thickening layers. These 

 pores or pits may be either circular (Fig. 84), elliptical, or elongated. 

 The pits in adjoining cells converge, and would form one continuous 

 canal, were it not that the unthickened primary cell wall persists as a 

 CLOSING MEMBRANE between two converging pits. As a result of 

 the continued thickening of the cell wall, the canals of several pits 

 often unite, and so branched pits are formed. Such branched pits 

 have usually very narrow canals, and occur for the most part only 

 in extremely thick and hard cell walls, as, for instance, in those of 



