CONIFER.^. 



449 



wall of the pollen-sacs is usually delicate, and finally dehisces longitudinally to 

 allow the escape of the pollen-grains, which are produced in extraordinarily large 

 numbers, since they have usually to be carried by the wind to the female organs 

 of the same or of another tree. The pollen-grains which happen to fall on the 

 opening of the micropyle of the ovules are retained by an exuding drop of fluid, 

 which about this time fills the canal of the mycropyle, but afterwards dries up, 

 and thus draws the captured pollen-grains to the nucleus, where they immediately 

 emit their pollen-tubes into its spongy tissue. In the Cupressinese, Taxineae, and 

 Podocarpeae this contrivance is sufficient, since the mycropyles project outwardly ; 

 in the Abietineae, where they are more concealed among the scales and bracts, 

 these themselves form, at the time of pollination, canals and channels for this 

 purpose, through which the pollen-grains arrive at the micropyles filled with fluid 

 (c/. Strasburger, /. c). The large number and lightness of the pollen-grains enables 

 them to be carried great distances by the wind ; in the true pines and the Podo- 



' Fik.. :!-i — . ( pollen-praiii of Thuja orieitlalis before its escape from the pol!en-sac, / fresh, //, /// after lying in 



water, the extine e having been stripped off by the swelling of the intine i; B pollen-grain of Pinits Pinaster before 

 its escape, e the extine with its vesicular protrusions bl. 



carpex their capacity for transport is increased by the vesicular hollow protrusions 

 of the extine, as represented in Fig. 321, /F, F, bl. 



The Mode of cell- division i?i the interior of the pollen-grain of Coniferce, to 

 which allusion has already been made, is still but imperfectly known; especially 

 now that we know more, through Millardet's researches, of the male prothalhum of 

 Selaginella and Isoetes, it is greatly to be desired that fresh observations should 

 enable us to compare with them the corresponding structures in Coniferse. Schacht 

 asserts that in Taxus, Thuja, and Cupressus, only one division-wall (Fig. 321 A) 

 arises at right angles to the longest diameter of the pollen-grain; one of the daughter- 

 cells is much smaller than the other, and the larger of the two developes into the 

 pollen-tube. In Larix, Pinus, Abies, and Podocarpus, two daughter-cells of very 

 different size are also first of all formed. But the septum between them arches 

 into the cavity of the larger one, and the protuberance (the papilla of the smaller 

 cell) is cut off by a partition ; a third cell is thus formed lying in the cavity of the 

 larger of the two primary cells, grows at its apex, and again divides. A three- or 

 four-celled body (or row of cells) is thus formed in the cavity of the pollen-grain, 



Gg 



