EXPLANATIONS OF THE PLATES. 593 



others in its interior ; under this is a second of the same kind, and below this is a 

 third containing two free cytoblasts. 



Fig. 12. A condition somewhat later than fig. 1 1. a. The encasing of the cells 

 in one another is very evident. 



Fig. 13. A part of the living hair. Reticulated streamlets proceed from the 

 cytoblast in all the cells, but they are drawn in only two of the cells. 



Fig. 14. The first commencement of the formation of an embryo in Pedicidaris 

 palustris : a, b. and c, d. display the outline of the ernbryo-sac in the region of the 

 penetrating pollen-tube, which at the end, in the embryo-sac, is swollen globularly, 

 and contains two very young cells and a free cytoblast ; below in the pedicel there 

 are three oval, free cells. 



Fig. 15. Very early condition of the embryo of Sagittaria sagittcefolia. It may 

 be here seen how the pollen-tube, loose at the beginning, is gradually formed into 

 a little cellular body by the continued formation of cells in cells. 



Fig.16. First commencement of an oil-duct in the tubers of Georgina variabilis. 

 In a single cell of the parenchyma, two free cells have been formed, which have 

 already separated between them a large drop of oil, and are characterised by their 

 cytoblasts. 



Figs. 1724 : see 1819. 



Fig. 17. Porous cell-walls in Abies excelsa: a. transverse section. The two 

 primary cell- walls are clearly separated (Hartig's Eustathe), and the deposit- 

 layers are penetrated by the pore-canals (Hartig's Astathe and Ptychode). b. The 

 longitudinal section : it displays at c. the simple wall of the cell, at d. the double 

 wall of this and the neighbouring cell. 



Fig. 18. A delicate section from the wall of the spiral-fibrous cells in the leaf 

 of Oncidium (cdtissimum ?). The spiral fibres appear separated from the original 

 cell-wall by a sharp line. The bark (external layer) of the fibre is not to be dis- 

 tinguished in the thinner and younger fibres, b. 



Fig. 19. A similar section, as in fig. 18, from the leaf of Vanda teretifolia, but 

 upon the double wall there are two spiral fibres lying one on the other. The 

 external layer of the individual fibre is at least as thick as the doubled wall of the 

 two cells. 



Fig. 20. A similar section of an annular vessel in the stem of Arundo Donax. 

 The external layer of the annular fibre is at least three times as thick as the 

 original cell-membrane. The internal portion of the spiral fibre was treated with 

 warm caustic alkali, it swelled up, and became gelatinous without changing the 

 original cell-wall, or the external membrane of the spiral fibre. 



Fig. 21. In the gynophore of Magnolia grandiflora, after flowering, the cells 

 are very thick, and consist of several layers which are traversed by branched 

 pore-canals. The figure displays two of these projecting one against the other. 

 The pore-canals pass through the layers of the cell often at right angles. 



Fig. 22. The pore-canals as above are seen to represent a very thickened cell 

 from the bark of Fraxinus excelsior var. jaspidea. 



Fig. 23. Two cells from the tuber of Georgina variabilis with delicate spiral 

 fibres. 



Fig. 24. A cell from the root membrane of Oncidium altissimum, with delicate 

 spiral fibres, which in some places separate from each other. These spaces ap- 

 pear at a later period to produce an actual perforation of the primary cell-wall. 



PLATE II. 

 Figs. 1 6: Siliceous Shield of Navicula viridis ( 82.). 



Fig. 1. Anterior view. In the middle line are two clefts, each about half the 

 whole length, and terminating at the centre, as well as at the other ends, with a 

 little circular enlargement. This is seen more clearly in figs. 3. and 4. Above, 

 below, and in the middle, upon the anterior and posterior surface of the shield, 



Q Q 



