MORPHOLOGY 



. 65 



TION. The later growth in thickness of most cell walls takes place by 



apposition, and thus the stratification, Avhich such thickened walls 



exhibit, is brought about (Fig. 66). Thicker, dense layers alternate 



with thinner less dense ones. The denser 



layers can be recognised by their high re- 



fractive power. In many cases lamella, 



deposited by apposition, become further 



thickened and otherwise modified by a 



process of intussusception. Three distinct 



layers can frequently be distinguished in 



strongly thickened cell walls, such as those 



of the wood, a primary, a secondary, and 



a tertiary thickening layer ; these differ in 



their optical appearance and their chemical 



composition. The secondary thickening 



layer is usually the most strongly devel- 



oped, and forms the chief part of the cell 



wall. The tertiary or inner layer is usually 



more highly refractive. 



Fl - 6s. -Surface view of cells from 

 the sensitive side of the tendril 

 of Cucurbita Pepo< showill , ta ,. 



tile pits, s. (x 540.) 



Thicker cell walls or layers of the wall which 

 appear homogeneous frequently exhibit a stratifica- 

 tion when treated with strong acids or alkalies. In 

 many cases the thickening layers exhibit delicate 

 striations in surface view. The striations extend through the whole thickness of 

 the layers, usually running obliquely to the long axis of the cell, and often crossing 

 one another in the different thickening layers (Fig. 67). A similar appearance of 

 crossing of striations may result from the single thickening layers of two adjoining 

 cells being visible at once. In special cases, but only in the formation of repro- 

 ductive cells, an inner thickening layer, completely 

 detached from the others, is produced, as in the 

 formation of pollen grains and the spores of 

 Bryophytes and Pteridophytes, which, enclosed 

 only by this inner membrance, 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. 



-phe thickening of the cell wall seldom 



Bio. 69. T,a,,sv,rs, section thro^h 

 similar ceils to those in Fi K . 08; a 



small crystal of calcium oxalate is , , < i ,111 < 



present in thetactiiepits. (x4so.) takes place uniformly over the whole surf ace; 

 but some portions are thickened, while, at 



other points, the original cell wall remains thin. In this way pores 

 are formed which penetrate the thickening layers. These pores 

 or PITS may be either circular (Figs. 66 w, 71 m), elliptical, or 

 elongated. The pits in adjoining cells coincide, and would form one 

 continuous canal, were it not that the unthickened primary cell wall 

 persists as a PIT MEMBRANE between the two pits. The openings 

 of narrow elliptical pits into adjoining cells usually appear to cross 



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