5H 
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 
Taxineae 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 Pi?ius. [The process of fertilisation 
takes place, according to Strasburger ^, as follows. In 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 Salishiiria (Gingko biloba) 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, 1878.] 
