5^4 PHANEROGAMS. 



nucellus, put out their tubes at first only for a short distance into its tissue ; their 

 growth is then for a time suspended. After the archegonia are completely de- 

 veloped, the pollen-tubes begin to grow again into the endosperm in order to 

 reach them'. This interruption of their growth lasts, in those Coniferse whose 

 seeds ripen in a single year, for only a few weeks or a month; when the seeds 

 take two years to ripen, as in Juniperus sibirica and communis, and Pinus sylvestris 

 and P. Strobus, it lasts until June of the next year. Whilst the pollen-tubes pene- 

 trate through the loose portion of the tissue of the nucellus, their width gradually 

 increases at their lower end, their wall becoming at the same time thicker ; until at 

 length they meet the wall of the embryo-sac which has now become soft, break 

 through it, penetrate into the funnel of the endosperm mentioned above, and attach 

 themselves firmly to the cells of the neck of the archegonia. In the Abietineae and 

 Taxinese each pollen-tube fertilises only one archegonium ; and several tubes there- 

 fore penetrate into the funnel at the same time ; in the Cupressineae on the contrary 

 one pollen-tube suffices for the fertilisation of the whole group of archegonia beneath 

 the broad funnel of the endosperm. The tube entirely fills up the funnel and applies 

 itself to the necks of the whole group of archegonia; short narrow protuberances 

 from the wide pollen- tube now grow into the separate necks of the archegonia, 

 forcing the neck-cells from one another and destroying them, and at length reaching 

 the oosphere. The same process takes place in the Abietineae and Taxineae; the 

 pollen-tube, after widening, becomes narrower and enters the neck of only one 

 archegonium, and penetrates finally as far as the oosphere. A thin spot may be 

 observed at the extremity of this protuberance of the thick-walled pollen-tube, which 

 obviously facilitates the escape of the fertilising substance ; and this is probably 

 assisted by the pressure exerted by the tissue which lies above on the part of the 

 pollen-tube outside the archegonium. Hofmeister states that a few free primordial 

 cells (Fig. 355, /) are sometimes formed in the end of the pollen-tube, which he was 

 inclined to consider as rudimentary indications of mother-cells of antherozoids 

 (corresponding somewhat to those in Salvinia), and Strasburger has detected the 

 existence of bodies of this kind in Juniperus and Pinus. [The process of fertilisation 

 takes place, according to Strasburger^, as follows. \xi Juniperus virginiana he ob- 

 served that when the apex of the pollen-tube reached the archegonia, the primordial 

 cells in the end of the tube arrange themselves so that they lie over the archegonia. 

 These cells now undergo absorption, the more anterior ones disappearing first. He 

 frequently detected in the oosphere of Picea^ at the time of fertilisation, two nuclei. 

 One of these is the nucleus of the oosphere, and may be termed the ' female pro- 

 nucleus ;' the other appears to have passed into the oosphere from the pollen-tube, 

 and is the 'male pronucleus' {spermakern). These two nuclei coalesce to form the 

 definitive nucleus of the oospore. It seems that, in addition to the nuclear sub- 

 stance, a certain amount of protoplasm also passes into the oosphere from the 

 pollen-tube, and fuses with the protoplasm of the oosphere. The pollen-tube of 

 the Abietineae contains a considerable number of starch-grains, and these too are 



* In Salishtiria {GingT<o hiloba) fertilisation does not take place until October, when the seed is ripe 

 and has already fallen off. The embryo is developed within the seed during the winter months. (See 

 Strasburger, Die Coniferen und Gnetaceen, 1872, p. 291.) 



2 [Ueber Befruchtung und Zelltheilung, 18/8.] 



