84 



BOTANY AND PHARMACOGNOSY. 



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. 



Fig. 54. Development of pollen grains (microspores) of garlic (Allium narassiflorum): 

 a, pollen mother cell with nucleus; b, the same with homogeneous nucleus and a thicker 

 wall; c-e, changes in nucleus prior to division; f, formation of spindle with nuclear masses 

 in the center from which nuclear threads extend to the poles of the spindle; g, division of 

 nuclear substance and receding of it from the center of the cell; h-i, further stages in the 

 organization of the nuclear substance at the poles; k, formation of a wall between two 

 daughter cells; 1, beginning of division of one daughter cell; m-n, final divisions resulting 

 in the formation of a tetrad (group of 4 cells). — ^After Strasburger. 



The Microsporangia (pollen sacs) arise by the division of 

 certain cells under the epidermis of the anther (Fig. 53). This 

 process of division continues until four regions of fertile tissue 

 (sporangia) are produced (Fig. 53, D). The sporangia are 

 directly surrounded by a continuous layer of cells which consti- 

 tutes the tapetum or tapetal cells (Fig. 53, 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 

 of the anther and the discharge of the pollen, and this layer is 

 called the endothecium (Fig. 53, end). There is still a third or 



