In comparing the female prothallium of the gymnosperms with that of the fern group we see a remarkable change has taken place. The female prothallium of the gymnosperms is very much reduced in size. Especially, it no longer leads an independent existence from the sporophyte, as is the case with nearly all the fern group. It remains enclosed within the macrosporangium (in cycas if not fertilized it sometimes grows outside of the macrosporangium and becomes green), and derives its nourishment through it from the sporophyte, to which the latter remains organically connected. This condition of the female prothallium of the gymnosperms necessitated a special adaptation of the male prothallium in order that the sperm cells may reach and fertilize the egg-cell. [Illustration: Fig. 374. Gingko biloba. _A_, mature pollen grain; _B_, germinating pollen grain, the branched tube entering among the cells of the nucellus; _Ex_, exine (outer wall of spore); _P_₁, prothallial cell; _P_₂, antheridial cell (divides later to form stalk cell and generative cell); _P_₃, vegetative cell; _Va_, vacuoles; _Nc_, nucellus. (After drawings by Hirase and Ikeno.)] [Illustration: Fig. 375. Gingko biloba, diagrammatic representation of the relation of pollen tube to the archegonium in the end of the nucellus. _pt_, pollen tube; _o_, archegonium. (After drawing by Hirase and Ikeno.)] =636. Gymnosperms are naked seed plants.=—The pine, as we have seen, has naked seeds. That is, the seeds are not enclosed within the carpel, but are exposed on the outer surface. All the plants of the great group to which the pine belongs have naked seeds. For this reason the name “_gymnosperms_” has been given to this great group. [Illustration: Fig. 376. Spermatozoids of zamia in pollen tube; _pg_, pollen grain; _a_, _a_, spermatozoids. (After Webber.)] [Illustration: Fig. 377. Spermatozoid of zamia showing spiral row of cilia. (After Webber.)] =637. Classification of gymnosperms.=—The gingko tree has until recently been placed with the pines, yew, etc., in the order _Pinales_, but the discovery of the spermatozoids in the pollen tube suggests that it is not closely allied with the Pinales, and that it represents an order coordinate with them. Engler arranges the living gymnosperms somewhat as follows: Class Gymnospermæ.