34 



MORPHOLOGY OF THE CELL. 



form a firm framework, the outer masses of layers become lignified ; in other cases, 

 on the other hand, the outer, rarely the inner layers, are transformed into mucilage. 

 Usually an inner layer of the cell-wall remains unchanged in all three cases, enabling 

 the above-mentioned cellulose reactions to be recognised, while the corky and lignified 

 scales of the cell-wall may, by previous treatment with alkalies or with nitric acid, also 

 exhibit these reactions ; the layers which are transformed into mucilage are the most 

 refractory. 



Some of the morphological processes here treated of find their explanation only 

 when we observe the formation of tissues ; but I cannot enter on an account of the 

 chemical behaviour of the cell- wall ; the reactions here mentioned must properly be 

 regarded not as chemical tests, but only as the means of recognising the morphological 

 differentiation. The description of some examples will be sufficient to direct the 

 beginner. 



The pollen of Thunbergia alata 

 (Fig. 38) shows that the different 

 development of two systems of 

 layers of a cell -wall may go so 

 far that the cuticularised layer 

 (here called extine) becomes ac- 

 tually separated from the non-cuti- 

 cularised shell (here designated in- 

 tine) which still possesses the power 

 of growth ; and that by this means 

 it becomes broken up by previ- 

 ously formed fissures in most cases 

 into one or two spiral bands. This 

 can be artificially induced by laying 

 these pollen-grains in concentrated 

 sulphuric acid or potash ; the extine 

 then assumes a very beautiful red 

 colour, while the intine in the fif^t 

 case dissolves, in the second case 

 swells a little and remains colour- 

 less. In the germination also of 

 many spores {e.g. Spirogyra, Mosses, 

 &c.) the cuticularised exospore be- 

 comes completely separated and 

 stripped from the endospore, which 

 still continues to develop ; both 

 shells, however, corresponding to 

 the extine and intine of the pollen-grain, consist, in their actual development, of systems 

 of layers of a single cell-wall possessing a different chemico-physical constitution. 



In the epidermis-cells, the cuticularisation either affects a shell of the outer wall, or 

 it attacks the side-walls, as may be seen, for instance, in a very exquisite manner on 

 the under-side of the leaf- veins of the holly. If a very thin transverse section (Fig. 

 39, A) is treated with Schultz's solution, and submitted to a very high magnifying 

 power (800), each cell- wall of the epidermis appears to be composed of two shells, of 

 which the inner, softer, and more capable of swelling (c) becomes dark blue, while 

 an outer shell does not assume this colour. But this latter shows itself even further to 

 be composed of two chemically different layers, an inner {b), which assumes a yellow 

 colour and penetrates laterally between the cells {b'), and an outer one which remains 

 colourless {a), and extends continuously over the cells (the so-called true cuticle). 

 Between these two may be observed yet another boundary zone, which, when the 

 microscope is exactly focussed, passes over it like a shadow. The inner shell, which 



Fig. 38.— Pollen of Thioibergia alata (X5S0). /and // placed in con- 

 centrated sulphuric acid ; IV, V, yil also after solution of the intine ; some- 

 times the fissures of the extine run so that isolated pieces of it fall off, cor- 

 responding to the lids of the extine of other pollen-grains, c.£-. of Cucurbita ; 

 ///in Schultz's solution ; Vf in strong solution of potash, e extine, t intine. 

 The fissures of the extine clearly arise by subsequent inner differentiation 

 in the same manner as the elaters are formed from the so-called special 

 mother-cell of the spores of Equisetum. (Cf. Book II. EquisetaceK.) 



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