POLLEN. 



[ 518 ] 



POLLEN. 



(which then become furrows) when dry, but 

 swell out, and display the latter clearly when 

 placed in water or dilute acids (PI. 32. 

 figs. 18 & 20). When the so-called pores 

 exist, they are either like simple pores 

 (PL 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 thick- 

 ening layers of the young pollen- grain, 

 inside the parent-cell, exhibit pits (exactly 

 comparable to those of ordinary pitted cells) 

 at the places corresponding to the future 

 pores, and curiously enough, in some cases 

 at least, the pits of adjacent pollen-cells cor- 

 responded, although in the mature expanded 

 compound grains they were far separated. 

 Sometimes the lids are found at the end of 

 short projecting processes (PI. 32. fig. 22). 

 The pollen of (Enothera and allied genera 

 exhibit remarkable conditions which have 

 been mistakenly described. The form of the 

 grain is that of a depressed sphere, with three 

 large equidistant 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 distin- 

 guishable (PL 32. fig. 14). The outer coat 

 thins off towards the end of each process. It 

 appears 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 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 (PL 32. fig. 22) and other cases. 



In Mimulus moschatus (PL 32. fig. 24) the 

 slits or furrows are curved, and in Nymphcea, 

 Pinus, and other cases, still more complex. 

 It 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 other 

 portions falling upon nectaries or secreting 

 surfaces) swell slightly, and germinate, as it 

 were, sending out a delicate tubular process 

 from one or more of the so-called pores or 

 slits (PL 32. fig. 30), which processes (the 



pollen-tubes) insinuate themselves between 

 the loosely packed cells of the stigma, and, 

 continually elongating, make their way down 

 the style and along the conducting tissue to 

 the ovules. In the Coniferae the pollen- 

 grains fall directly upon the micropyle of 

 the naked OVULE, and send their pollen- 

 tubes into it. The pollen-tube is produced 

 by the growth of the inner or proper coat of 

 the pollen into a tubular filament. When 

 pollen- grains are placed in dilute sulphuric 

 acid or in syrup (sometimes in water), they 

 absorb liquid, swell, and their contents partly 

 exude from pores, &c., either to a slight ex- 

 tent, as a little " hernia," as it were, of the 

 inner membrane, or in large quantity in a 

 worm-like, irregular mass ; in the latter case 

 the coagulation of the surface often produces 

 a pellicular coat. These exuded masses are 

 of course distinct from the true pollen-tubes 

 produced under natural conditions. 



The fluid contents of the pollen-grains 

 consist of a granular viscid protoplasm, with 

 minute starch-granules and (apparently) oil- 

 drops, making together what has been called 

 the fovilla, which increases in density as 

 the pollen ripens. The starch-granules ex- 

 hibit molecular motion in the pollen-tube, 

 and especially when they escape by rupture. 

 The granular contents of the pollen-cell, 

 which are always rendered opaque by the 

 action of water, are gradually transferred to 

 the pollen-tube as it elongates. 



Connected with this point is the pecu- 

 liarity exhibited by the pollen of the Coni- 

 ferae. fn the Abietineae the form of the 

 granules is very peculiar, elongated, curved, 

 and with bulging ends; and, according to 

 Schacht, a distinct internal cell exists, at- 

 tached at one side in the cavity of the ordi- 

 nary pollen-cell, this internal cell dividing 

 and growing out as the pollen-tube when 

 the pollen-grain comes upon the ovule. 

 The pollen of the Cupressineas is spheroidal, 

 but free cellules appear to be formed in the 

 pollen-tubes during the fertilization. These 

 conditions, which are not yet satisfactorily 

 cleared up, indicate a relation to the sper- 

 matozoid-producing spores of the Marsilea- 

 cese, &c., analogous to that between the Gym- 

 nospermous ovules and the ovule-spores of 

 those Cryptogamic families. 



It has been imagined that the form and 

 structure of the pollen-grains might have 

 some relation to the general structure of the 

 plants, and might serve as an indication of 

 systematic position and affinities. But there 

 appears to be no definite relation ; very va- 



