CONIFERJE. 



457 



before the access of the pollen-tube to the endosperm) ; this cell being clearly 

 equivalent to the canal- cell so often mentioned in Vascular Cryptogams which is 

 afterwards converted into mucilage \ In Abies canadensis and excclsa and Finns 

 Larix this canal-cell is, according to Strasburger, very evident ; while in the 

 Cupressineae (Thuja, Juniperus, and Callitris), its demarcation from the rest of the 

 contents of the central cell is only slight. As in those Vascular Cryptogams where 

 the ventral part of the archegonium is plunged in the tissue of the prothallium, 

 the neighbouring cells become transformed by further divisions into a parietal layer 

 surrounding the central cell, so the same thing takes place also in the endosperm 

 of Coniferce. In the Abietineag each archegonium is separated from an adjacent 

 one by at least one, often by a large number of layers of cells : those of the 

 Cupressincce, on the other hand (Fig. 325, cf), are in lateral contact. The arche- 

 gonia of Taxus are short ; in those of the Abietineoe the central cell is elongated ; 

 in the Cuprcssinese it becomes angular from the pressure of the adjacent cells. 

 The number of the archegonia which are formed in the endosperm beneath the 

 apex of the embryo- sac is very various; Hofmeister and Strasburger state that in 

 the Abietineoe it is from three to five, in the Cupressineae from five to fifteen 

 (according to Schacht it may even be thirty) ; in Taxus baccata from five to eight. 

 The continuous growth of the surrounding endosperm causes the formation of 

 funnel-shaped depressions above the archegonia, which in some Abietinese are but 

 shallow, in Finns Finaster^ F. Slrobus, &c., deep and narrow. In these species each 

 of the funnel-shaped depressions leads down only to the neck of one archegonium ; 

 in the Cupressineae (Callitris, Thuja, and Juniperus), where they lie closely crowded 

 together, the cluster of them is walled round by the endosperm, and a funnel is 

 formed common to them all, which still remains closed by the cell-wall of the 

 embryo-sac. 



Fertilisation. The pollination of the ovules takes place before the archegonia 

 are formed in the endosperm ; the pollen-grains, having reached the apex of the 

 nucleus, 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 Coniferae 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 Finus sylvestris 

 and F. Strobus, until June of the next year. Whilst the pollen-tubes penetrate through 

 a loose portion of the tissue of the nucleus, 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 therefore 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 



' In Figs. 324 and 325, which are transferred from the first edition, the canal-cell is not 

 indicated. 



