44 THE CELL-WALL. 



additional material between the original particles, a process which has been termed 

 "intussusception." 



The appearance of stratification in thickened cell-walls is naturally most strik- 

 ing where substances of different kinds have been deposited alternately in the 

 different parts of the wall, and when successive layers take up unequal quantities 

 of water. The thickening may at length result in such an extreme restriction of 

 the cell-cavity that its diameter is less than that of the inclosing wall. Sometimes 

 nothing remains of the cavity but a narrow passage, and then the cells are like 

 solid tibres. Formerly they would not have been classed with cells at all, but 

 would have been distinguished under the name of tibres, from the forms resembling 

 honey-comb cells. The protoplasts in these contracted cells languish and often die, 

 especially when the walls of the self-made prison are greatly thickened and do not 

 allow of intercourse with the world outside. But generally a protoplast takes care, 

 in constructing its dwelling, not to close itself in entirely, nor to cut itself off 

 permanently from the outer world. It either makes from the very beginning little 

 windows in the walls of its house, leaving them quite open or closed only by thin, 

 easily-permeable, membranes; or else, after constructing a completely closed enve- 

 lope, it redissolves a piece of it, thus making an aperture through which in due 

 time it is able to effect its escape. The scope of this work does not admit of an 

 exhaustive treatment of the formative power possessed by protoplasts needful for 

 these results; it will be sufficient to give a general description of some of the 

 more important processes which have for their object the establishment of a 

 connection between adjacent cell-cavities and of communication with the external 

 world. 



The new particles of material, or cellulose, which are to strengthen the 

 delicate original cell-membrane, are in many instances not deposited or intercalated 

 evenly over the entire surface of the protoplast. Little isolated spots are left 

 unaltered, and these may be compared in a way to the small glazed windows in a 

 living-room, or cabin port-holes closed by thin panes of glass. The part of the 

 thickened wall which immediately surrounds the little window, and which so to 

 speak constitutes its frame, has, besides, often a very characteristic structure, 

 being elevated so as to form first a ring -like border, and eventually a hood, 

 arching over the window and perforated in the middle (see fig. 10 x ). A comparison 

 of this structure, arched over the thin spots in a cell-wall, to the iris spread in 

 front of the crystalline lens in an eye would be still more appropriate. A similar 

 annular border projects likewise from the window - frame on the other side, 

 facing a neighbouring cell-cavity, so that the window appears symmetrically 

 vaulted on both sides by mouldings with round central apertures (fig. 10 2 ). 

 Supposing someone wanted to pass from one cell-cavity to the other he would have 

 in the first place to go through the hole in the moulding on his side. He would 

 then find himself in a roomy space, which we will call the vestibule, and would 

 next have to break through the little window, which is somewhat thickened in 

 the middle, but elsewhere is as soft and thin as possible. On the further Bide 



