POLLEN. 



[ 617 ] 



POLLEN. 



other portions falling upon nectaries or 

 secreting surfaces) swells slightly, and ger- 

 minates as it were, sending out a delicate 

 tubular process from one or more of the so- 

 called pores or slits (PI. 40. fig. 30), which 

 processes (the pollen-tubes) insinuate them- 

 selves 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 development 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.j either to a slight extent, 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 

 exhibit molecular motion in the pollen- 

 tube, and still more clearly 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- 

 ferse. In the Abietinese the form of the gra- 

 nules 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 Cupressineae is spheroidal ; 

 but free cellules appear to be formed in the 

 pollen-tubes during the f ertilizatiou. 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 

 Gymnospermous 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 varied pollen occurs within the limits 

 of the same family, and very similar pollen- 

 grains in families widely distant. There 

 appears, however, to be a certain relation 

 within the limits of genera. It may be per- 

 haps generally stated that the Monocotyle- 

 dons have frequently one pore or furrow ; 

 the Grasses often three pores, as is the case 

 with many Dicotyledons, many of which 

 have more, while a large number of the 

 families of the latter division exhibit both 

 pores and slits. As microscopic objects, it 

 is most convenient to class the forms arti- 

 ficially, or according to structure ; and we 

 give a brief list of the principal varieties 

 arranged under this point of view. 



The pollen-grains of Zostera, Zanichellia, 

 and other submerged aquatic plants, have 

 no cuticle or outer coat ; all other known 

 forms possess one or more outer layers. 



A. Outer coat without furrows or pores. 



a. Outer coat granular : Strelitzia Regince, 

 Calla palustris, Crocus sativus, fyc. f 

 Asarum ewop&um, Laurus nobilis, fyc., 

 many Euphorbiaceae. 



b. Outer coat with papillae : Canna indica. 



c. Outer coat with cell-like reticulations : 

 Ruettia formosa (PI. 40. fig. 23), R. 

 strepens, Tribulus terrestris. 



In Periploca grceca (PI. 40. fig. 15) and 

 Apocynum venetum (fig. 7) grains of this 

 kind are connected in fours in one plane ; 

 in some Luzulee tetrahedrally. 



B. Outer coat presenting longitudinal fur- 



rows (or folds). 



* One furrow (the form of most Mono- 

 cotyledons). 



a. Outer coat finely granular : common 

 in Monocotyledons among the Dico- 

 tyledons, in Myrica ceri/era. Magnolia 

 grandiflora, Liriodendr'on tulipiferum, 

 $c. 



b. Outer coat granular, spiny : Nympluea 

 alba. 



c. Outer coat with cell-like reticulations : 

 Hemerocallis fulva, and other Monoco- 

 tyledons. 



