376 XXVII. REPRODUCTION OF GYMNOSPERMS. 



The reproductive nucleus which passes into the oosphere has 

 therefore the value of a spermatozoid, and indeed in Zamia, 

 Ginkgo biloba (Salisburya adiantifolia), and Cycas revohita, all 

 Gymnospermous Phanerogams, large multiciliate spermatozoids 

 have actually been discovered. In general we may say that the 

 reproductive nucleus of the pollen-tube is distinguished only in 

 the details of form from the spermatozoid of a Vascular Crypto- 

 gam, while the want of motility is Compensated for .by the 

 growth of the pollen-tube right up to the embryo-sac. In both 

 cases the nucleus forms the main body of the fertilising mass. 



Development of the Embryo, The next stages take the em- 

 bryo nucleus into the end of the oospore remote from the neck, 

 where, by repeated bipartition, it forms four nuclei lying in 

 the same plane (Fig. 137, D). These are separated from one 

 another laterally by partition walls. They repeat their bipartition 

 transversely to the long axis of the oospore, and become separated 

 off from one another in this direction also (E). The four nuclei 

 lying at the end of the oospore again divide in the same direction, 

 and the resulting nuclei which now lie nearest to the end of the 

 oospore once more divide. Ultimately therefore, at that end 

 of the oospore which is farthest from the neck, we find three 

 tiers, each of four cells, and above these, in the general body of 

 the oospore, four free nuclei (F). These free nuclei enlarge very 

 considerably, and later on sink. Of the three tiers of cells, that 

 which is nearest to the neck remains as a four-celled rosette 

 at the base of the archegonium, the median ones elongate, forming 

 the " embryonal tubes," from which the suspensor is formed, and 

 carry the cells which were most remote from the neck of the 

 archegonium down into the tissue of the prothallium (G). These 

 last cells constitute the rudimentary embryo. They are distin- 

 guished from the beginning by their richer contents, and soon 

 divide into two (so early as G), and then into three stages. 



We can use the same fir, Picea vulgaris, Lk., in order to study 

 older ovules with rudimentary embryos. We can either use the 

 material fresh, and at intervals of about eight days, or the 

 material can at similar intervals be laid in alcohol. The rudi- 

 mentary embryo rapidly increases in size and in number of cells 

 by the formation of periclinal, anticlinal, and radial walls, and 

 takes the form of the adjoining Fig. 138, A. These divisions 

 preclude from the very first the existence of an apical cell. After 

 the embryo has further enlarged, its hinder end begins to develop 



