0)1 the Structure of Roofs. 469 



It is a jjeneral character of all plants which produce flowers, 

 that they possess all the kinds of organs of vegetation, i. e. stern, 

 root, and leaf, in a rudimentary form, at the time when they 

 become independent of their parent. They are reproduced bv 

 seeds, properly so called, the principal distinctive character of 

 which is, that they contain an embryo, or infant plant, composed 

 of the vegetative organs above mentioned, in a more or less 

 developed state. If we remove the skin of a turnip-seed, we find 

 within an embryo, on which we can distinguish two expanded 

 laminae, representing leaves, called the cotyledons or seed-lobes, 

 attached upon a stalk curved over upon the seed-lobes, and 

 attenuated at the other end; tlie attenuated end is the radicle or 

 rudimentary root, the intermediate portion is the rudimentary 

 stem, and if we separate the two cotyledons, we find it ter- 

 minating at its upper end in a minute leaf-bud, the point whence 

 the future upright stem will rise. In this case, as in all the 

 Cruciferae (or cabbage family), the embryo forms the whole seed 

 excepting the coat or skin. The same is the case in the seeds of 

 beans, pease, and other leguminous plants. In many cases a 

 similar embryo of smaller size lies imbedded in a mass of sub- 

 stance of horny, fleshy, or floury consistence : an external pro- 

 vision of food, called endospej-m, provided for the same purpose as 

 the substances contained in the large cotyledons of the turnip, 

 bean, &c., namely, to feed the embryo plant in germination, 

 before its root has acquired the power of drawing nourishment 

 from the soil. Tliis is the structure of the seed of flax, carrots, 

 parsnips, beet, &c. 



The arrangement, such as we have just described, does not 

 prevail, however, in all flowering plants, nor even in all those 

 with which we have concern in the present paper. In fact, the 

 two primary divisions of flowering plants forming seeds in closed 

 seed-vessels, are characterized most strikingly by the diversity in 

 the structure of their seeds. All those presenting the characters 

 above indicated arc combined under the name of Dicotyledons, 

 or plants with tico seed-lobes, and separated from the Monocoty- 

 ledons, or plants with one seed-lobe to the embryo. Of the latter 

 class we have examples in the seeds of the corn-plants, and otljer 

 grasses, in the onion tribe, &c. In them the rudimentary stem 

 is not surmounted by a pair of cotyledons with the bud between 

 them, but a single cotyledon exists, folded round the upper part 

 of the stem, and enclosing the bud much in the same way as the 

 leaves of young grass-plants are rolled round and enclose the 

 bud which afterwards throws out the ear. 



This diversity of plan in the seed does not at first sight seem 

 to be of much practical importance. But it is connected with 



