260 ORGANOGRAPHY. 



Development and Structure. — We have already seen (p. 248), 

 that tlie pollen corresponds to the cellular tissue of the leaf situ- 

 ated between the epidei'mis of the upper and lower surfaces. 

 It has also been stated, that the pollen was formed in certain 

 cells developed originally in the centre of the parenchyma of 

 the young anther {fig. 500, cm); also that these cells were en- 

 (jlosed in a special covering of their own {fig. 500, c/), and 

 that in the course of growth they pressed upon the surrounding 

 parenchyma, ci. so as to cause its more or less complete ab- 

 scn-ption, and finally assisted in promoting the dehiscence of the 

 anthei'. We have now more particularly to describe the mode of 

 development and the structure of the pollen. 



The formation of the pollen has been already described under 

 the head of Cell-division loith absorption of the walls of the 

 parent cell, and the setting free of the new cells (see p. 60). We 

 will however recapitulate the principal points connected with 

 this subject. The large cells ( fig. 500, cvC) which are de- 

 veloped in the parenchyma of the young anther and which are 

 destined for the formation of the pollen, are called parent or 

 mother cells : the primordial utricle of each of these becomes 

 infolded as in ordinary cell-division so as to divide it into four 

 portions, either directly, or indii'ectly by first dividing it into 

 two, and then each of these being again divided into two others; 

 these four portions are called special parent or mother cells; the 

 whole of the protoplasmic contents in each cell then secretes 

 a layer of membrane on its outside, and we have thus four 

 perfect cells {fig. 144, a, b, c, d) formed in the cavity of the 



parent cell, which constitute the 

 Figs. 548. and 549. true pollen-cells; as these pro- 



gress in development, one or 

 more layers are foi-med Avith a 

 few exceptions on the outside of 

 their original membrane, which 

 ultimately exhibits markings 

 and projections of various kinds 

 (figs. U, 553, &c.), the nature 

 of which varies in the pollen of 

 different species, but is always 

 the same in any i)articular one. 

 The four pollen-cells continuing 

 Fig. 548. ToUen of Peripioca ffrceca. ^ increase in sizc, distcnd the 



rig. M9. FoUen of Iiigu ano- „ , , . i 



mala. After Jussieu. parent ccU and ultunatcly causc 



its absorption; and subsecpicntly 

 the special mother cells are generally absorbed also, by Avhich 

 the pollen-cells or grains are set free in the locuhis of the 

 anther in tlieir ])crfctt condition. Sometimes the membrane 

 of the si)ecial parent cells is not completely absorbed, in which case 

 the pollen -grains of the parent are more or less connected, and 



