CHAPTER XXXV 

 GERMINATION AND DORMANCY 



The Structure of Seeds. — All seeds contain an embryo plant which is 

 enclosed by one, or more commonly by two, seed coats.^ The seed coats 

 originate from the integuments of the ovule and often exhibit external struc- 

 tural evidences of this origin even in the mature seed. Among these are the 

 hilum, which represents the place where the seed was attached to the ovule 

 stalk {fu?iiculus), and micropyle, which frequently persists in the mature 

 seed, and the raphe, a remnant of the ovule stalk which in certain kinds of 

 seeds is adherent to the seed coats. When a single seed coat is present it is 

 usually hard and woody but when there are two seed coats the inner is almost 

 invariably thin and membranous. 



The embryos in seeds of different species of plants differ markedly in 

 size and appearance but in every case a mature embryo possesses one or more 

 cotyledons, a plumule, and a hypocotyl (Fig. 13?) • The cotyledons are the 

 seed leaves and they vary in number from one in the monocots to as many 

 as fifteen in the embryos of some conifers. The embryos of dicots have two 

 cotyledons, as the name implies. Structurally the cotyledons are modified 

 leaves but usually they differ greatly in appearance from the foliage leaves 

 of the same species. The cotyledons (or cotyledon) are attached near the 

 upper end of the short thick stem-like axis of the embryo, the hypocotyl. 

 The plumule or bud of the embrj^o is usually located just above the point at 

 which the cotyledon or cotyledons are attached to the hypocotyl. The plumule 

 consists of a meristem with several rudimentary foliage leaves. The primary 

 root of the plant develops from the lower end of the hypocotyl. The rudi- 

 mentary root at the lower end of the hypocotyl is often called the radicle. 



An endosperm is also present in the seeds of many species (Fig. 137, D). 

 This tissue develops from the endosperm nucleus and usually contains con- 

 siderable quantities of accumulated foods. In the seeds of those species which 

 contain no endosperm, such as the legumes, the cotyledons are usually en- 



^ In the skunk cabbage {Spathycma foetida) and possibly in a few other 

 species the seed consists only of a naked embryo (Rosendahl, 1909). 



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