44 THE CEJ>L-\V.\1.:.. 



ii(l(litii)iiiil material between the original j)articles, a process which has lieen ternieil 

 " intussusception." 



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

 ing where substances of ditlerent kimls have been deposited alternately in the 

 dittereut parts of the wall, and when successive layers take up ini<'(|ual ipiantities 

 of water. The thickening may at lengtli 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 naiTow passage, and then the cells are like 

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

 would have lieen distinguished under the name of fibres, from the forms resembling 

 liouey-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 

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

 in constructing its dwelling, not to close himself in entirely, nor to cut him.self ofl" 

 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 eff"ect 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 ol>ject the establishment of a 

 connection between adjacent cell-cavities and of connnunication 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 tlie entire surface of the p)i'otoplast. 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 in a 

 manner represents its frame, has, besides, often a very characteristic structure, 

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

 perforated in the middle and ai'ching over the window (see fig. 10^). A compai'ison 

 of this structure, curved over the opening in a cell- wall, to the iris spread in 

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

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

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

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

 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 side 



