THE WALLFLOWER 13 O 



and analogous tissues from a fancied resemblance to 

 the white of an egg. The Wallflower seed is commonly 

 described as exalbuminous ; but this is not strictly 

 true, for we know from recent researches that one 

 layer of endosperm at least is persistent. In many 

 plants the embryo-sac becomes completely filled with 

 endosperm, the embryo itself occupying only a small 

 space. Sucli seeds are said to be albuminous. Fig. 48 

 shows a young embryo-sac filled with endosperm, from 

 the Elder. 



h. The Ripe Seed 



We will now complete the description of the seed, 

 which is the ripened ovule, with its contents. The 

 nucellus, as we have seen, disappears in great part 

 even before fertilisation. By the time the seed is 

 ripe scarcely a trace of it remains except a little group 

 of cells at the chalaza, in contact with the tips of the 

 cotyledons of the embryo. The integuments, on the 

 other hand, remain (though some of their layers are 

 obliterated), and develop into the hard seed-coat or 

 testa, which when the seed is ripe consists of several 

 layers of cells (see Fig. 49,). On the extreme out- 

 side is a layer of clear cells with very thick walls, so 

 thick that there is no cell-cavity left at all. These 

 walls are mucilaginous, and swell up in water, render- 

 ing the seed-coat sticky, and causing it to adhere to 

 the soil or to anything which it touches. Beneath 

 this are two or three layers of brown cells, more or 

 less flattened, and on the inside of the seed-coat 

 next the embryo is a layer of thin-walled cells, with 

 very abundant contents, consisting chiefly of nitro- 

 genous food-materials in the form of proteid granules 



