DICOTYLEDONOUS SEED OF CAPSELLA. 411 



are strongly flattened, and thickened almost to the obliteration of 

 the cavity. If we examine the spermoderm from the outside, we 

 easily recognise the contour of the polygonal cells of the external 

 tabular layer. These cells have their more internal portions 

 partly separated by intercellular spaces full of air. In the middle 

 of each cell is to be distinguished a weakly-defined, more strongly- 

 refractive portion. The walls of the next inner layer of cells are 

 brown, strongly thickened the cells themselves only a little 

 smaller than in the outer layer. Considerably smaller, on the 

 other hand, and weakly thickened, are the cells of the aleurone- 

 containing layer. 



If we now allow water to run into the section from under 

 the edge of the cover-glass, we see the cells of the outer layer 

 rapidly swell ; each bulges strongly outwards ; at its centre a 

 highly-refractive column is noticeable. The cell-cavity is not 

 distinguishable ; the entire cell appears filled by the thickening 

 layers of the wall, and in all cases the outer thickening layers are 

 feebly, the inter highly, refractive. These internal thickening 

 layers form the striking central column, which now projects very 

 strongly on the surface of the seed, while simultaneously the 

 intercellular spaces between the cells disappear. The swelling 

 walls usually show clear lamination. With further addition of 

 water, the cuticle of the cells is ruptured, and the outer thickening 

 layers come out, diffusing in the surrounding water as invisible 

 mucilage. The refractive column remains behind, marking the 

 centre of each cell (Fig. 149, B, at e). It has increased not 

 inconsiderably in size ; at its apex can be seen the relics of 

 the dissolved thickening layers. In the same way the lateral 

 middle lamellae of the cells remain, and, as they do not swell, 

 show now a much less height than the columns. All this can 

 be seen in our Fig. 149, B, which represents the testa after 

 the action of water. These phenomena of swelling can be ob- 

 served more quickly if the section is first examined in alcohol, and 

 then water run in. This mucilagination of the thickening layers 

 of the outer cells of seeds and mericarps l is a comparatively 

 common phenomenon, which facilitates the fixing of the seed 

 in the soil ; and, on the other hand, results in the firm retention 

 of water on the surface of the seed. 



1 Mericarps, the segments of fruit which, like the Pelargonium (Geranium), 

 Cranes' Bills, and Parsley Worts, do not dehisce to let out their seeds, but 

 split up bodily into seed-containing segments. Hence these fruits are called 

 schizocarps splitting fruits. [ED.] 



