POLLEN. 



[ 616 ] 



POLLEX. 



antecedent process of solution, tlie grains 

 maj- 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 

 ■w ith a number of others of the same gene- 

 ration by the persistence of the Avails 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- 

 piadacefe. 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 slighth', and may readily be sepa- 

 rated. This is not the case, however, with 

 the majoiity 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 

 (tigs. 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. AVe have observed that the outer 

 coat is often deposited as a very thick layer 

 inside the special parent cell, and that, when 

 the latter dissohes, 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 oi Helk'bo7-its &c. (see Epideemls), 

 is altogether unknown ; probably all the 

 cases are referable to one cause. 



It has been mentioned that the u re 

 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 

 p;)llen when di'y, the coat collapsing at 

 the thin places, so that gi-ains of this 

 kind appear oval or angular, not clearly ex- 

 hibiting the slits (which then become fur- 

 7'otvs) ; but they swell out and display the 

 latter clearly when placed in water or di- 

 lute acids (PI. 40. figs. 18 & 20). AN'hen 

 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 quatemate pollen) 

 we have seen the thickening layers of the 

 young pollen-grain, inside the parent cell, 

 exhibit iiits (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 the end of short projecting 

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

 GEnothera 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 couical 

 masses; and two laminoe are distinguishable 

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

 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 hai-dened and, 

 after circumscissile fission, pushed oft" as a 

 plate by the advancing pollen-tube, instead 

 of giving way and expanding, we should 

 have the lid occurring in Cncitrbita 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 

 ^'lowering plants. When scattered from 

 the anthers, that portion of tlie pollen 

 which falls upon the stigma (and frequently 



