178 MINUTE STRUCTURE OF THE SEED. 



THE SEED. 



502. The ripened ovule 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 morphologi- 

 cally to the prirnine and secundine of the ovule ; the outer being 

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

 the inner the tegmeu, or eudopleura. The nucleus of the 

 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 an} r sclerotic tissue, 

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

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

 Ia3 - ers, which are not constant in their characters. 



504. The ordinary layers met witli in the seeds of most 

 agricultural plants have been described by Nobbe l in the follow- 

 ing terms : 1 . The hard la3 T er, composed generally of palisade 

 or staff-like cells of considerable firmness. In Leguminosse 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 the}' are placed in water. This layer is sometimes found 

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

 pigment Ia3"er, 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 less color. In a few other 

 instances the color is not dependent on a pigment laj'er ; for, as 

 Frank 2 has shown, in the steel-blue seeds of species of Paeonia 

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

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

 seeds are dark red or dark brown ; hut when thoroughly moist- 



1 Haudbuch <ler Samenkunde, p. 73. - Botanische Zeitung, 1867. 



