430 PART IV. CLASSIFICATION. 



varies from a small number (3-5) iii the Abietinea^ to a large 

 number (20-60) in Welwitschia and Gnetum. The archegonia 

 are either scattered (Abietinese), or in a group (Cupressinese) : 

 when scattered, the central cells are surrounded by a layer of 

 small cells belonging to the prothallium ; when in a group, the 

 central cells are in actual contact and have a common investment 

 of small cells. 



The female cell or oosphere is a relatively large nucleated cell, 

 the protoplasm of which is so highly vacuolated that it presents a 

 frothy appearance. 



Fertilisation. When the microspore has reached the apex of the 

 nucellus, it developes a pollen-tube which penetrates the tissue of 

 the nucellus, making its way to the archegonia which have been, 

 or are being, developed on the prothallium inside the macrospore. 

 Generally speaking, the pollen-tube, on reaching the macrospore, 

 pierces its wall, and enters the neck of an archegonium (when 

 scattered), or spreads out over the necks of a group of adjacent 

 archegonia ; a male cell is forced out through the mucilaginous tip 

 of the pollen-tube into the oosphere, or into each of the oospheres 

 of a group of archegonia so that one male organ fertilises several 

 archegonia ; the act of fertilisation is completed by the fusion of 

 the male pronucleus with the female pronucleus, to constitute the 

 nucleus of the oospore (Fig. 249). 



With regard to those forms in which the male cell is a sperma- 

 tozoid, in Cycas and Ginkgo the pollen-tube does not approach the 

 archegonia, but by its growth causes the absorption of much of the 

 micropylar portion of the nucellus, so that a cavity containing 

 liquid is formed, into which the spermatozoids are discharged. In 

 Zamia the pollen-tube comes into contact with the neck of the 

 archegonium, into which it discharges the two spermatozoids 

 together with a drop of liquid. In all cases the spermatozoids 

 swim down the neck of the archegonium to the oosphere, and one 

 of them enters it and fertilises it. 



The Results of Fertilisation. 



1. The fruit. In all the Gymnosperms which have a cone-like 

 macrosporangiate flower (Cycadacese, except Cycas ; Coniferse, 

 except Taxe3e), one effect of fertilisation is to cause more or less 

 considerable growth in the macrosporophylls, or in the placental 

 scales, as also tissue-change resulting in their becoming woody 

 (e.g. Pinus, Abies, etc.) or fleshy (e.g. Juniperus), the product being 

 the fruit. 



