336 



ORGANOGRAPHY. 



Fig.Ha. Germinating 



cotyledons in their embr3^o, are called Dicotyledonous. 'But there 

 W^n 7±f{ is another class of plants in which there is 



commonly but one cotyledon present (fffs. 

 745 and 689, c), and which are, accord- 

 ingly, termed Monocotyledono^is. Sometimes, 

 however, a monocotyledonous embryo has 

 more than one cotyledon, in which case the 

 second cotyledon alternates with the first, 

 instead of being opposite to it, as is invari- 

 ably the case with the two cotyledons of 

 dicotyledonous plants. By the difference thus 

 presented in the embryos of Flowering Plants, 

 these plants are divided into two great classes, 

 called respectively Dicoti/ledojis and Monoco- 

 tyledons. The spore of Flowerless Plants, 

 having no true embryo, can have no cotyle- 

 dons, and hence such plants are called Aco- 

 tyJedonous. Hence we have primarily two 

 great divisions in the vegetable kingdom : 

 namely, Cotyledonous and Acotyledonous Plants ; 

 the former being again divided into Monoco- 

 tyledons and Dicotyledons. The structure of the 

 spore, and other peculiarities connected with 

 enibryo of'ti'i'e Oat'. Acotyledonous Plants, will be described here- 



tiirouSif'^heTths^ ^'^'^^^ (^^^ P- ^^^^- ^^' ^^^® "'^^' therefore, 

 CO. c. Cotyledon, g. Only to alhide to the embryo of Dicotyledons 

 Young stem. ^^(j Monocotyledons. Before doing so, how- 



ever, we must say a few words as to the development of the embryo. 



Development of the Embryo. — AVhen 

 the process of fertilization has been 

 effected, the embryo-sac, as already 

 noticed, becomes filled with a aiass 

 of loose cells destined for the sup- 

 port of the embryo, and which are 

 developed from the protoplasmic mat- 

 ter contained in its interior. The 

 embryo is thus furnished with mate- 

 rials necessary for its growth ; and it 

 accordingly commences an active de- 

 velopment. 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 par- 

 titions, by which it is converted into a string of cells of varying 

 length, which forms the stispensor or cord by which the embryo 

 is at first suspended from the embryo-sac (see Reproduction of 

 Anqiospermia). The terminal cell of this body continues to 

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



Fig. 746. Progressive develop- 

 ment of a dicotyledonous 

 embryo. 1. Earliest stage. 

 2, 3. Stages of progression. 

 4. Most developed. 



