34 



MORPHOLOGY OF THE CELL. 



In the epidermal cells, the cuticularisation either affects a shell of the outer wall, 

 or it attacks the side-walls, as may be well seen, for instance, on the under-side of 

 the leaf-veins of the holly. If a very thin transverse section (Fig. 37, J) 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 one, which 

 is softer and more capable of swelling (c), becomes dark blue, while the outer shell 

 does not. But this latter shows itself to be further composed of two chemically different 

 layers, an inner (h), 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 focussed to it, passes over the 

 field of view like a shadow. The inner shell which assumes the blue colour, as well 

 as the outer cuticularised substance, are each composed of a system of layers. In 



the latter moreover the radial stria- 

 tion is more evident, as is shown 

 in Fig. 37, ^, «, ^ ; these radial lines 

 are not, as was formerly thought, 

 pores, but are the transverse sec- 

 tions of layers which, in a front 

 view of the cuticle (Fig. 37, B, j), 

 appear as striae, and, following the 

 veins of the leaf lengthways, pass 

 over the septa of the cells (q). 



An example of strongly lignified 

 cell-wa'.ls split up into three shells 

 occurs in the dark-brown-walled 

 sclerenchymatous cells which com- 

 pose the firm bands between the 

 fibro-vascular bundles in the stem 

 of Pteris aquilina (Fig. 38). The 

 very thick wall between two cells 

 contains a hard, dark-brown lamella 

 {a) in the centre of the double cell- 

 wall; this is followed on each side 

 by a fight-brown, more horny shell 

 {b) ; and this encloses a third shell 

 likewise fight-brown. By boifing in 

 nitric acid with potassium chlorate 

 the first {a) is dissolved, and the 

 cells are thereby isolated (see Fig. 

 28, p. 28) ; the two other shells of the cell-wall {b and c) remain unchanged by the 

 maceration, except that they lose their colour; and hence the shell c is shown to 

 be composed of different layers, some more and some less watery (Fig. 28, C, c). The 

 three shells also show a different behaviour on treatment with concentrated sulphuric 

 acid; a becomes a dark reddish brown and does not swell, or only slightly; b swells 

 in the radial direction and becomes thicker ; while c swells in the radial, tangential, 

 and longitudinal directions (see Fig. 38, C, c, and D, c) ; in transverse sections c breaks 

 away from b and curves in a vermiform manner (C) ; in longitudinal sections it is bent 

 in a wavy manner (D). 



In true wood-cells, e.g. in Pinus syl'vestris (Fig. 24, A, p. 25), three shells are likewise 

 generally to be distinguished ; one in the centre of the double cell-wall {A, m), next 

 a thicker one {%), and then an innermost (z) ; the two first turn yellow on treatment with 

 solution of iodine or iodine and sulphuric acid, the innermost blue with the latter 

 reagent ; z and i are dissolved by concentrated sulphuric acid, while the central lamella 



Fig. 36.— Pollen of Thunbtrgia alata (X 530). / and // placed in 

 concentrated sulphuric acid; IV, V, VII after solution of the intine ; 

 sometimes the fissures of the extine run so that isolated pieces of it fall 

 off, corresponding to the lids of the extine of other pollen grains, e.g. of 

 Cucurbita ; III in Schultz's solution, section ; VI in strong solution of 

 potash, e extine, i intine. The fissures of the extine clearly arise from 

 subsequent internal differentiation, in the same manner as the elaters are 

 formed from the so-called 'special mother-cells' of the spores oi IZqiiise- 

 turn, (See Book II., Equisetaceae.) 



