MINUTE STKUCTURE. 



5.'J 



gen and oxygen, and to this compound tlic name alhtlosc 

 has been given. 



We have said that the protoplasm is the active principle 

 through tlie agency of which all the vital processes of 

 the plant are carried on. It contains at some time or 

 other every constituent of the plant. The cell-wall is 

 itself, therefore, a product or .tpcrctton of the protoplasm, 

 and is at first an extremely thin lilm, wiiich, however, 

 gradually increases in thickness by the addition of further 

 material. This new material is deposited bdwcfn ttir 

 molecules of the original film, and so extend.s not only 

 the surface of the wall, but, by deeper deposits, the 

 thickness also. This process of acquisition of new mate- 

 rial is known as intussusception. 



As the wall between two cells increases in thickness, a di.stinct 

 middle layer is discernible in it, known as the middle 

 lamella (m, Fig. 2(i0). Tiiis portion of the common wall 

 is diff(>rent in chemical composition from the rest, so that 

 it may, tinder jiroper treatment, be dissolved and the 

 the cells thereby separated. 



It is in the earlier stages of their history, while the walls are 

 comparativeh' thin, that the cells possess the greatest 

 activity. Bj' these alone is carried on the process of 

 growth, which consists in the multiplication and enlarge- 

 ment of cells. 



It is seldom the case that the wall is thickened unifoi-mly. 

 Often numerous round thin spots are left, so that the cell 

 has a dotted appearance (Fig. 2(>1). When the thin 

 spots in adjacent cells are contiguous, as they commonly 

 are, a readj' means of intercommunication is afforded. 

 Sometimes the spots, instead of being round, are oblong, 

 so that the cell under the microscope presents a ladder- 

 like appearance, and so is said to bo scalarifonn (Fig. 

 2(52). Then, again, the thickening may take the form of 

 sjyiral bands upon the inner surface ; or, instead of a 

 continuous spiral band, we may find a series of isolated 

 rings, when the marking is said to be annular. Jietieu- 

 lated cells are also found, in which the markings, as the 

 name implies, form a sort of network on the walls. 

 Several of these forms aresliown in Figs. 2G3 and 2(>4. 



Sometimes round thin spots will be left in the wall, and over 

 each of these a thick-walled dome with an oi)ening at 

 the top will be formed. At the same time a similar dome 

 is raised at exactly' the same spot on the other side of the 

 wall in the next cell; and, finally, the thin partition 

 between the opposite domes break awaj', permitting free 

 communication. Thus are formed what are called bor- 

 dered pit.'i (Fig. 265), which abound in the wood of Coni- 



fe>f 1 



^i'^ 



Fif?. 200. 



FIk. SGI. 



d 



— 



:? 



Fitr. 263. Fig. 264. 



Flgr.265. 



