GYMNOSPERMAE. CONIFERAE. 



333 



A 



FIG. 259. Callitris quadrival-vis. Longitudinal section through 

 the ovule, A slightly, B highly magnified, ynt integument, Nu 

 nucellus, Sp the sporogenous cell-group, the cells'of which however 

 are all sterile with the exception of one which becomes the ma- 

 crospore (embryo-sac). B shows the portion of Fig. A marked Sp 

 more highly magnified ; t tapetal cells. 



sporium undergoes further divisions and a group of cells is developed from it, as in 

 the Cycadeae, a sporogenous tissue therefore, the cells of which however, with the 

 exception of one which produces the macrospore or embryo-sac, are sterile and are 

 displaced by the growing macrospore. Here too the prothallium or endosperm is 

 formed entirely within the macrospore. The nucleus of the latter divides, and by 

 frequent repetition of the process a number of free nuclei are produced round which 

 cells are formed, and these soon unite 

 laterally and grow in a radial direction, 

 and divide in such a manner that the 

 macrospore becomes filled with the pa- 

 renchymatous tissue of the prothallium. 

 The archegonia, formerly termed 

 corpuscula \ are formed at the apex of 

 the macrospore from single superficial 

 cells of the prothallium, in exactly the 

 same manner as in other higher Arche- 

 goniatae. The mother-cells swell and 

 are densely filled with protoplasm, and 

 divide by a transverse wall parallel to 

 the surface of the macrospore (Fig. 

 260 //). In this way a large inner 

 cell is formed, the central cell of the 



archegonium, and an upper smaller cell (Fig. 260 II K) lying against the wall of the 

 macrospore, from which the neck of the archegonium proceeds. The neck remains 

 simple and one-celled in Abies canadensis and elongates to an extent corresponding 

 with the increase in size of the prothallium; but in most cases the original neck-cell 

 divides into several cells which either lie in one plane or form several tiers one above 

 another, as in Picea excelsa and Pinus Pinaster. The neck-cells seen from above 

 appear as rosettes of four or, as in Picea excelsa, of eight cells. A ventral canal-cell 

 is also formed by the separation of a small portion of the contents of the large central 

 cell beneath the neck from the remaining part, the oosphere, after preliminary division 

 of the nucleus. This happens in the Abietineae shortly before fertilisation, in 

 Juniperus virginiana and others after the appearance of the pollen-tube; in these 

 species the canal-cell is very soon disorganised and may be easily overlooked. The 

 cells of the tissue of the prothallium which surround the central cell of the arche- 

 gonium in the Coniferae develope by further divisions into a parietal layer round the 

 central cell, as in the Cycadeae and other Archegoniatae. In the Abietineae each 

 archegonium is separated from its next neighbour by at least one and often by many 

 layers of cells ; those of the Cupressineae on the contrary are in contact with one 

 another laterally (Fig. 262). The archegonia of Taxus (Fig. 261) are very short; in 

 those of the Abietineae the central cell is elongated, but in the Cupressineae it is 

 rendered angular by the pressure of the adjoining cells. The number of archegonia 

 formed at the apex of the prothallium varies greatly ; in the Abietineae, according to 

 Hofmeister and Strasburger, there are from three to five, in the Cupressineae from 



1 [See note on page 300.] 



