Ch. VIII, 2] MORPHOLOGY OF SEEDS 373 



(page 356). Far more common, however, are the simple 

 brown and gray colors such as all ripening tissues assume, 

 as a purely chemical incident, where no functional reason 

 for special color exists. This is the case with all wind- and 

 water-disseminated seeds, and those thrown by spring 

 mechanisms. 



In texture, seeds are almost invariably hard, sometimes 

 extremely so, as in the Ivory Palm. The hardness results 

 from three causes : the almost complete absence of water 

 from the tissues, the very thick coats, and the frequent 

 large proportion of cellulose food. They soften very greatly, 

 however, in germination. 



Many seeds are difficult to distinguish externally from 

 small fruits, especially when the latter are one-seeded. 

 The difference is purely morphological, consisting in the 

 presence of the ripened ovary as a kind of additional coat; 

 but functionally such one-seeded fruits are identical with 

 true seeds. 



As with other plant organs, there are various structures, 

 popularly thought to be seeds which are not, such as the 

 little black, hard-walled bulblets in the axils of Lily leaves 

 (Fig. 212). There is no such thing as "Fern seed," the mis- 

 named "fruit dots" on the under sides of fern fronds con- 

 taining only spores, a very different kind of structure. Spores 

 have no embryo, that is, no ready-formed young plant, which 

 all true seeds contain. Indeed the possession of an em- 

 bryo is by far the most distinctive mark of a seed. 



2. The Structure, Morphology, and Functions of 



Seeds 



Despite a wide variety in their external features, seeds 

 possess in common certain primary parts, three and some- 

 times four in number, which of course are those essential 

 in their function. 



First in importance is the embryo, which is simply a 

 formed but unborn plant, with its development held for a 



