MODIFICATIONS OF CELLULAR TISSUE. 



63 



FIG. 23. ROUND 



LARLY IN CIRCLES. 



always remain so, but often acquires considerable hardness. 

 This is the case, for example, in the prickles of .the Rose and other 

 plants, which are merely connected with the cuticle, and are not 

 prolonged from the wood beneath. It is the case also in the 

 stones of the Plum, Peach, Cherry, c. ; and in the gritty matter 

 in tjie centre of the Pear. In all these parts, 

 the processes of vegetation are no longer going 

 on ; but the power of firm resistance is re- 

 quired in their place. This is effected, by the 

 deposition of solid matter within the cells. 

 Sometimes the new product lies in regular 

 layers, one within another, covering the whole 

 membrane ; sometimes it is deposited in what ARRANGED 



, , .... 



appears a less regular manner, certain points of 



the membrane being left uncovered by it. In this last case, 



however, an additional object is 

 attained; for the cells, though 

 the greatest part of their walls is 

 so much thickened, are still in a 

 degree permeable to fluid, through 

 the spots of the membrane on 

 which no deposit has taken place. 

 These spots in the walls of con- 

 tiguous cells generally correspond 

 with each other ; so that fluids 

 can find their way from one cell 

 into the cavities of the neighbour- 

 ing ones, though so large a pro- 

 portion of their contents has be- 



J 



COHie SOHd. When tllC Walls Of 



11 u v j.i_ J.L j 



cells have *>* thus strengthened 



j n particular parts, the membrane 

 has a dotted appearance ; the thin- 

 nest portions seeming almost like perforations. 



75. The size of the cellules of this tissue is extremely variable ; 

 they are usually from 1-300 to 1-500 of an inch in diameter; but 

 may be found of all sizes, from 1-30 to 1-3000 of an inch. One 



Fio. 24. SECTIONS OF CELLS STRENGTH- 



ENED BY INTERNAL MATTER IRREGULARLY 



DEPOSITED ; the shaded portion indi- 

 rates the remaining cavifies : a, cells 



from the gritty centre of the pear ; &, 



cells from the stone of the plum. 



