PRINCIPAL GROUPS OF PLANTS. 121 



The Development of the Two Generations, namely, the 

 sporophyte and gametophyte, is much the same in the Angio- 

 sperms as in the Gymnosperms ; that is, the sporophyte consti- 

 tutes the plant body and what is commonly considered to be the 

 plant. The gametophytes are still more reduced than was the 

 case in the Gymnosperms, the male gametophyte consisting of 

 but two cells. 



Beginning with the germination of the seed, we may outline 

 the life history of the plant as was done under Gymnosperms. 

 The seeds in the two groups are much alike, with the exception 

 that in the Angiosperms they usually have two integuments. 

 Within the Angiosperms two classes of embryos are distinguished, 

 which give rise to the most important division of this group of 

 plants. In the one case a single cotyledon is formed at the apex 

 of the stem, and all plants having an embryo of this kind are 

 known as MONOCOTYLEDONS, that is, plants having one seed leaf. 

 In the other case two cotyledons arise laterally on the stem and 

 opposite each other, and those plants having an embryo of this 

 type are grouped together as DICOTYLEDONS, or plants having two 

 seed leaves. In the monocotyledons the cotyledon is limited to one, 

 but in the dicotyledons the seed leaves are not limited in number 

 and there may sometimes be three or more. 



The sporophyte which develops from the germinating seed 

 consists of the essential parts already given, i.e., root, stem, and 

 leaves. The leaves are of four kinds: (i) Foliage leaves, (2) 

 scale leaves or bud scales, (3) floral leaves, which in some cases 

 are wanting, and (4) sporangial leaves or sporophylls. Inasmuch 

 as the latter give rise to the gametophytes (male and female) the 

 development of the sporangia in each will be considered in detail. 



The Microsporangia (pollen sacs) arise by the division of 

 certain cells under the epidermis of the anther (Fig. 79). This 

 process of division continues until four regions of fertile tissue 

 (sporangia) are produced (Fig. 79, D). The sporangia are 

 directly surrounded by a continuous layer of cells which consti- 

 tutes the tapetum or tapetal cells (Fig. 79, t), these being in the 

 nature of secretion cells and containing considerable oil. The 

 tapetum is in turn surrounded by a layer of cells which are 

 peculiarly thickened and which on drying assist in the opening 



