FERTILISATION IN GYMNOSPERMS. 755 



seeded Cycadeae the central cell is often 3-5 mm. long and 2-3 mm. broad. 

 The neck of the archegonium differs much in structure in the different forms of 

 Gymnosperms, but there can be no doubt whatever that these organs are in every 

 respect to be looked upon as archegonia resembling those of the higher Cryptogams 

 and Mosses. 



When the prothallium (endosperm) with its archegonia is sufiBciently developed, 

 usually only after some months, fertilisation can follow: the poUen-tube i bores 

 through the tissue of the nucellus of the ovule b, and applies itself with its anterior 

 end either as a thin tube, or more frequently as a broad sac, to that part of the 

 embryo-sac where the archegonia are situated : protuberances of the pollen-tube then 

 bore their way into the necks of the archegonia and penetrate as far as the central- 

 cells. It is not yet known in what way the male fertilising material now passes 

 over into the central cell of the archegonium, filled with the oosphere. It is not 

 certainly established whether an extremely fine actual opening in the membrane 

 of the pollen-tube facilitates the direct entrance of the fertilising protoplasm, 

 or whether the membrane remains closed and the fertilising substance diffuses 

 over as a true solution. 



It is for our purpose practically the same thing whether the whole of the proto- 

 plasm contained in the large central-cell of the archegonium is to be regarded as 

 the oosphere, or whether only a certain part of it answers to this designation. It 

 suflBces for us to know that in the lower portion of the central-cell e, cell-divisions 

 now take place by means of which two or three tiers are produced, each of four 

 cells placed cross-wise and close together, as in/I This is however, strictly speaking, 

 not as yet the inception of the embryo. On the contrary, these cells, at first very short 

 and discoid, grow out to long tubes {/'\ which bore into the endosperm-tissue 

 and become curved and twisted in the process. The cellsy situated at the ends of 

 the tubes, hitherto but little grown, and which previously occupied the basal region 

 in the archegonium — i. e. the one turned away from the neck — and which are 

 pushed out from the archegonium by the formation of the tubes, subsequently 

 give origin to the embryo g; this then goes on growing, with continual cell- 

 divisions, and finally displaces the rest of the parts and occupies the middle 

 of the endosperm. Among the many peculiarities in the s eed-formation of the 

 Gymn osperms, it is found that in some cases fe rtilisation~takes placgg^ and the 

 seeds even b ecome Tully ripened and fall, without the embryo having develop ed 

 further. I observed this remarkable fact in the autumn of 1868 in the case 

 of the plum-like seeds of the Japanese plant Gingko hiloba. On opening the ripe 

 fallen seeds in October I found apparently no trace of an embryo in them, and 

 therefore regarded them as unfertilised. On again examining the seeds after they 

 had been laid aside for two or three months, there was found in each of them a large 

 well-developed embryo, and the seeds all proved to be capable of germination. 

 Strasburger has investigated this fact in his great work on the Gymnosperms^ 

 and we now know that the Cycadeae also behave similarly. 



The ripened seeds of the Gymnosperms, especially those of the Cycads and 

 of the Gingko biloba just referred to, often bear very little resemblance to what is 

 elsewhere termed a seed in the Phanerogams, those of the last-named plants being in 

 the ripe state like small yellow plums or large cherries ; for the integument of 



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