POLLEN. 



[ 517 ] 



POLLEN. 



contents secrete a series of layers, which, in 

 the ordinary course, by the solution of the 

 primary walls of the special parent-cells upon 

 which they were applied, become the walls 

 of free cells, which constitute the simple 

 ordinary pollen-cells. These subsequently 

 increase in size, and their outer coat assumes 



Fig. 600. 



Fig. 601. 



Fig. 602. 



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

 ment. 



Magnified 100 diameters. 



its characteristic form and appearance, while 

 free in the chamber of the anther (figs. 600, 

 601, 602). 



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 influences 

 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 divi- 

 sion 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 con- 

 dition of the pollen-grains (PL 32. 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 antece- 

 dent process of solution, the grains may be 

 still developed in the same order and manner, 

 and remain connected in greater or smaller 

 masses or groups, each enclosed in its special 

 parent-cell, itself connected with a number 



of others of the same generation by the 

 persistence of the walls of the cells in which 

 the parent-cells were developed. This ex- 

 plains the compound pollen of the Acacias 

 (PI. 32. fig. 27), and in an excessive form, 

 the waxy pollen-masses which occur in the 

 Orchidacese and Asclepiadaceae. It is some- 

 times stated that the pollen-grains of these 

 compound forms are merely connected toge- 

 ther by the viscid substance remaining from 

 the solution of the parent-cells, but this 

 would render such cohesions indefinite in 

 character, instead of being regular; at the 

 same time it will be understood that the 

 solution may have advanced so far that the 

 grains merely hold together slightly, and 

 may readily be separated. This is not the 

 case, however, with the majority of compound 

 pollen-grains. When the pollen-grains be- 

 come free, theu* viscid secretions are pro- 

 bably referable to the dissolved parent-cells. 



The metamorphoses of the outer coat or 

 cuticle of the pollen-grain are very remark- 

 able and not yet at all understood; the 

 granulations (PI. 32. figs. 11, 12), spines (figs. 

 8, 9, 22, 26), reticulations (figs. 13, 23, 27, 

 28), &c., characterizing mature grains, make 

 their appearance in the interval between the 

 solution of the special parent-cells and the 

 bursting of the anther, while the pollen- 

 grains lie free within the latter; their pro- 

 duction is accompanied by a general growth 

 and expansion of the pollen-grain. We 

 have observed that the outer coat is often 

 deposited as a very thick layer inside the 

 special parent-cell, and that when the latter 

 dissolves, the outer coat of the pollen-grain 

 is also in a softened condition, and becomes 

 stretched by the expanding inner coat, 

 finally forming a comparatively thin layer on 

 the ripe grain (ex. gr. in Tradescantia). The 

 mode of origin of the markings, like those 

 on Spores and on the cuticle of Helle- 

 BORUS, &c. (see Epidermis), is altogether 

 unknown ; probably all the cases are refer- 

 able to one cause. 



It has been mentioned that the mature 

 pollen-grain exhibits pores or slits. We 

 believe they should rather be regarded as 

 thinner places in the outer membrane. Their 

 number and position varies much, as will be 

 indicated presently on referring to some of 

 the principal types of form of pollen. The 

 slit-like markings are generally accompanied 

 by a peculiar shrinking of the pollen when 

 dry, the coat collapsing at the thin places, 

 so that grains of this kind appear oval or 

 angular, not clearly exhibiting the slits 



