ORGANS OF REPRODUCTION. 



345 



(fig. 742, c, and 688, c), and which are, accordingly, termed 

 Monocotyledonons. Sometimes however, a monocotyledonous 

 embryo has more than one cotyledon, in which pj^^ --^o, 



case the second cotyledon alternates with the 

 first, instead of being opposite to it, as is in- 

 variably the case with the two cotyledons of 

 dicotyledonous plants. By the difference thus 

 presented in the embryos of Flowering Plants, 

 these are divided into two great classes, called 

 respectively Dicotyledons and Monocotyledons. 

 The spore of Elowerless Plants, having no true 

 embryo, can have no cotyledons, and hence 

 such plants are called Acotyledonous. Hence 

 we have primarily two great divisions in the 

 vegetable kingdom : namely, Cotyledonous 

 and Acotyledonous Plants ; the former being 

 again divided into Monocotyledons and Dico- 

 tyledons. The structure of the spore, and 

 other peculiarities connected with Acotyledo- 

 nous Plants will be described hereafter. We 

 have now, therefore, only to allude to the 

 embi-yo of Dicotyledons and Monocotyledons. 

 Before doing so, however, we must say a few 

 words as to the development of the embryo. 



Development of the Embryo. — When the 

 process of fertilization has been effected, the 

 embryo-sac, as already noticed, becomes filled 

 with a mass of loose cells destined for the 

 support of the embryo, and which are devel- 

 oped from the protoplasmic matter contained in its interior. 

 The embryo is thus furnished with materials necessary for 

 its growth ; and it accordingly commences an active devel- 

 opment. At first it is a nucleated cell, called the germinal 

 vesicle, which adheres to the apex of the embryo-sac; this 

 elongates downwards, and its interior is soon divided by 

 transverse partitions, by which it is converted into a string 

 of cells of varying length, which forms the suspensor or cord 

 by which the embryo is at first suspended from the embryo- 

 sac. The terminal cell of this body continues to increase 

 in size by the process of cell-division, and soon forms a 

 little rounded, or somewhat oval cellular body at the end of 

 the suspensor {fig. 743, 1). This cellular body continuing its 

 growth soon begins to alter in shape, and assume that of the 

 embryo, of which it is the early stage; thus, the upper extremity, 

 in contact with the suspensor, tapers somewhat and forma the 

 radicle, while the lower extremity gradually becomes divided 

 into lobes, Avhich, by increasing in growth, form the cotyledons : 

 the suspensor, during this gradual enlargement, dies away, and 



Fig. 7i2. Germi- 

 nating embrvo of 

 the Oat. r. Root- 

 lets comi ng through 

 sheaths, co. c. Co- 

 tyledon, g. Young 

 stem. 



