MERISTKM. 45 



walls of endosperm cells in some plants (Date, Sagus taedigera, 

 Phyteleplias) become after a time enormously thick, the cell- 

 cavities being still connected by means of " pits " which traverse 

 the walls of adjacent cells. The thickening takes place mainly 

 by the deposition of layer after layer of cellulose, but, as a rule, 

 other substances are also deposited which confer upon the walls 

 great toughness {Phytelephas). 



The cells of the pith, of some plants {Hoya carnosa) have 

 extremely thick walls, through which pass " pits," usually simple 

 iu nature. As a rule, however, adult pith-cells are thin-walled 

 (Samhicus) and contain nothing but air. 



Epidermal cells often possess, as has been seen (see supra) ^■ery 

 thick outer walls {Fiscum album, Holly), and at times layer after 

 layer can be distinguished ; in such cases treatment of the walls 

 with caustic potash usually results in a separation and swelling 

 of the cuticle, followed by a swelling of the layers of the outer 

 wall. The thickening of the walls of sclerenchymatous fibres 

 and wood elements also takes place mainly by accretion. 



The wall of the young cell is not, however, devoid of inter- 

 stices ; indeed the fact that salt molecules of different sizes can 

 penetrate into the cell through the wall, points definitely to the 

 existence of such interstices. Naegeli looked upon the cell-wall 

 US being constituted somewhat as follows : — 



i. The ultimate molecules (micellie) of cellulose have spaces between 

 them. Each micella is supposed to be surrounded by a watery envelope. 



ii. These molecules are again grouped into larger particles (tagmata) 

 between which larger spaces exist. Thus a sort of complex meshwork is 

 produced, which permits of the passage of certain substances. 



It is highly probable that some such structure is present in 

 the cell-wall of a young cell, and that molecules of salts can pass 

 through. In this connection, however, the study of root-hairs 

 offers an exjjlanation of the absorption of salts into the interior 

 of the cell, which cannot be arrived at by simply considering the 

 structure of the cell-wall. It is, in fact, highly probable that the 

 ectoplasm lining the inner aspect of the wall of the root-hair 

 exercises a selective capacity upon the absorption of salts in 

 solution from the soil, some salts being admitted to the exclusion 

 of others ; and as in the root-hairs, so in the young thin- walled 

 cells of a rudimentary tissue, although in this case the materials 

 supplied to the cell are, as a rule, not raw, but elaborated, the mole- 



