POLLEN. 



[ 616 ] 



POLLKX. 



antecedent process of solution, the grains 

 may be still developed in the same order 

 and manner and remain connected in greater 

 or smaller ma-ses or groups, each enclosed 

 in its special parent cell, itself connected 

 with a number of others of the same gene- 

 ration by the persistence of the walls of the 

 cells in which the parent cells were deve- 

 loped. This explains the compound pollen 

 of the Acacias (PI. 40. fig. 25), and, as an 

 excessive form, the waxy pollen-masses 

 which occur in the Orchidacese and Ascle- 

 piadaceae. It is sometimes stated that the 

 pollen-grains of these compound forms are 

 merely connected together 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 ad- 

 vanced so far that the grains merely hold 

 together slightly, and may readily be sepa- 

 rated. This is not the case, however, with 

 the majority of compound pollen-grains. 

 When pollen-grains do become free, the 

 viscidity of their surface is probably refer- 

 able 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. 40. 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 be- 

 tween the solution of the special parent 

 cells and the bursting of the anther, while 

 the pollen-grains lie free within the latter ; 

 their production is accompanied by a gene- 

 ral 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 (e. g. in Tra- 

 descantia). The mode of origin of the 

 markings, like those on SPORES and on the 

 cuticle of Helleborus &c. (see EPIDERMIS), 

 is altogether unknown ; probably all the 

 cases are referable 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 ac- 

 companied 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 ex- 

 hibiting the slits (which then become fur- 

 rows) ; but they swell out and display the 

 latter clearly when placed in water or di- 

 lute acids (PI. 40. figs. 18 & 20). ^Vhen 

 the so-called pores exist, they are either 

 like simple pores (PI. 32. fig. 10), or they 

 may be provided with little disk-like pieces 

 or lids, which fall off and leave them bare 

 when the pollen-tube is formed (figs. 13 & 

 22). In all cases, however, we believe that 

 the outer coat is extended over the whole 

 surface, and that the slits and dots are 

 merely thinner places ; moreover, in certain 

 cases (Leschenaultia, a quaternate pollen) 

 we have seen the thickening layers of the 

 young pollen-grain, inside the parent cell, 

 exhibit pits (exactly comparable to those of 

 ordinary pitted cells) at the places corre- 

 sponding to the future pores, and, curiously 

 enough, in some cases at least, the pits of 

 adjacent pollen-cells corresponding, although 

 in the mature expanded compound grains 

 they were far separated. Sometimes the 

 lids are found at trie end of short projecting 

 processes (PI. 40. fig. 22). The pollen of 

 (Enothera and allied genera exhibits remark- 

 able conditions, which have been mistakenly 

 described. The form of the grain is that of 

 a depressed sphere with three large equi- 

 distant truncated cones projecting pretty 

 nearly in the same plane. The outer coat 

 is thick, except at the ends of the conical 

 masses ; and two laminae are distinguishable 

 (PI. 40. fig. 14). The outer coat thins otf 

 towards the end of each process. It ap- 

 pears to us that the inner coat or true 

 pollen-membrane does not extend into the 

 processes at all, but is globular, and that a 

 semifluid deposit occupies the space between 

 the inner coat and the outer, in the cavity 

 of the tubular processes. Now, supposing 

 such a deposit to become hardened and, 

 after circumscissile fission, pushed off as a 

 plate by the advancing pollen-tube, instead 

 of giving way and expanding, we should 

 have the lid occurring in Cucurbita Pepo 

 (PI. 40. fig. 22) and other cases. 



Is has been stated that the pollen is the 

 agent of fertilization of the ovules in the 

 Flowering plants. When scattered from 

 the anthers, that portion of the pollen 

 which falls upon the stigma (and frequently 



