178 MINUTE STliUCTURE OF THE SEED. 



THE SEED. 



502. The ripened ovnle is the seed. In ripening, the ovule 

 undergoes changes in the structure both of the integuments and 

 the nucleus. The integuments of the seed answer morpliologi- 

 call}' to the primine and secundine of the ovule ; the outer being 

 the testa, or seed-shell, — also called spermoderm or episperm, 

 — the inner the tegmen, or endopleura. The nucleus of tlie 

 seed also answers to the nucleus of the ovule. The morpho- 

 logical relations of the different parts of the seed have been 

 sufficiently treated in the first volume, "Structural Botany," and 

 therefore only the histological features will now be presented. 



503. Considered as a whole, the testa varies greatly in con- 

 sistence ; it is in some cases as dense as any sclerotic tissue, 

 while in others it is pulpy, and in others still, membranaceous. 

 But it is usually divisible under tlie microscope into two or more 

 layers, which are not constant in their characters. 



504. The ordinary la3'ers met witii in the seeds of most 

 agricultural plants have been described T)y Nobbe '■ in the follow- 

 ing terms: 1. The hard laj'er, composed generally of palisade 

 or staff-like cells of considerable firmness. In Leguminosae it 

 is the external layer, and its exposed surface is cuticularized. 

 In flax and species of Brassica, it is the second, in cabbage 

 and mustard, the third layer. In a few cases the cells of this 

 layer are tabular instead of staff-shaped. 2. The mucilaginous 

 layer, not present in all the common agricultural seeds, is com- 

 posed of cells whose walls have the power of swelling greatly 

 when they are placed in water. This layer is sometimes found 

 in the outer part of the testa, sometimes in the inner. 3. The 

 pigment layer, which imparts characteristic colors to the coats of 

 the seeds of many plants, is not constant in the form of the cells. 

 The color may reside in the cell-wall, or in the dried contents of 

 the cell. Sometimes a few pigment-cells are scattered among 

 others of a neutral tint, and even among those which cannot be 

 said to have any proper color at all. In some cases one of the 

 other layers may contain more or lesg color. In a few other 

 instances the color is not dependent on a pigment layer ; for, as 

 Frank '^ has shown, in the steel-blue seeds of species of Pseonia 

 the color is purely a result of reflected light, and is in no wise 

 due to the presence of any true coloring-matter. The dried 

 seeds are dark red or dark brown ; but when thoroughly nioist- 



1 Handbiich (lev SameiikunJe, p. 73. ^ Botanisclie Zeitiing, 1867. 



