POLLEN. 



[ 615 ] 



POLLEN. 



Fig. 592. 



Fig. 593. 



Cl 



Fig. 594. 

 CL Ci CM 



Vertical sections of a cell of a yonng anther of the 

 Melon, showing the gradual separation of the regions. 

 CE, epidermal cells ; CI, cells of the wall of the anther; 

 Ci, cells lining the loculi; CM, cells from which the 

 pollen is developed. 



Magnified 100 diameters. 



change then takes place : the contents of 

 each cell secrete a layer of cellulose, which 

 does not adhere to the wall of the parent 

 cell to form a layer of secondary deposit, 

 but lies free against it, so that a new free 

 cell is formed within each old one, nearly 

 filling it. The walls of the old cell (form- 

 ing a connected parenchymatous tissue) 

 then dissolve, so that the new cells be- 

 come free, no longer merely in their parent 

 cells, but in a cavity which is to constitute 

 the pollen-chamber or loculus of the anther. 

 These free cells are the parent cells of the 

 pollen of authors. A new phenomenon soon 

 occurs in these. These parent cells divide 

 into four by ordinary cell-division, either 

 by one or by two successive partings by 

 septa at right angles to each other but both 

 perpendicular to an imaginary axis (as when 

 an orange is quartered), or by simulta- 

 neously-formed septa which cut off por- 



tions in such a manner that the new cells 

 stand in the position of four cannon-balls 

 piled into a pyramid (tetrahedrally ). These 

 new cells are the special parent cells of the 

 pollen j and in each of these the entire pro- 

 toplasmic contents secrete a series of layers, 

 which in the ordinary course, by the solu- 

 tion of the primary walls of the special 



Fig. 596. 



Fig. 597. 



Pollen-grain of the Melon in various stages of de- 



velopment. 



Magnified 100 diameters. 



parent cells upon which they were applied, 

 become the walls of free cells, which con- 

 stitute the simple ordinary pollen-cells. 

 These subsequently increase in size ; and 

 their outer laminae assume the characteristic 

 form and appearance while free in the 

 chamber of the anther (figs. 595-597). 



In referring the peculiarities of many 

 kinds of pollen to circumstances connected 

 with the development, it may be noted in 

 the first place, that the mode of division of 

 the parent cells into quarters often influ- 

 ences the ultimate form of the pollen-grain : 

 thus, when the division is by two planes at 

 right angles, the original form of the pollen- 

 grain will be elongated, and the ripe grain 

 will probably be elliptical, while, when the 

 division is " tetrahedral," the grains may 

 retain the form thus produced, or be slightly 

 modified and become polygonal, or, as is 

 more common, they expand more readily 

 than the others into a sphere. But there is 

 no absolute rule here ; we find even the 

 tetrahedral and the polar division occur 

 together among the parent cells of the same 

 anther. In the next place, a compound 

 condition of the pollen-grains (PI. 40. figs. 

 7, 17) is readily explicable by referring it to 

 an arrest of the process of subdivision ; so 

 that if the walls of the special parent cell 

 do not dissolve, the pollen-grains will be 

 left in groups of four ; and if the parent 

 cells do not become singly detached in the 



