[Illustration: FIG. 250.--_I_ Pollen-grains of _Cupressus_; at the top is seen one prothallium-cell. _II_ Germinating; _c_ pollen-tube; _a_ the extine; _b_ the intine.] 2. =The Macrospores.= The prothallium in _Salvinia_ and _Marsilia_ is still rather large, green, and capable of the independent assimilation of carbon. It projects more or less from the macrospore and bears (in _Marsilia_ only one, in _Salvinia_ several) archegonia, which however are embedded to a greater degree in the prothallium, and are more reduced than the archegonia of the true Ferns and Horsetails (Figs. 215, 216). The prothallium is still more reduced in _Isoëtes_ and _Selaginella_; _partly_ because it is smaller and is in a higher degree enclosed in the spore, it also contains less chlorophyll, or is entirely without chlorophyll, and in consequence incapable of independent existence, whilst the number of archegonia is less; and _partly_ because the archegonia are themselves reduced, the cells of the neck are fewer and embedded to the level of the surface of the prothallium without any, or with only a very slight projection (Figs. 235, 236).--Finally, the prothallium with its archegonia begins to develope in _Selaginella_ while the macrospore is still within its sporangium, and before it is set free from the mother-plant. After the spores are set free and germination has commenced, the spore-wall ruptures and the prothallium is exposed. [Illustration: FIG. 251.--Longitudinal section of ovule of _Abies canadensis_. Inside the integument (_i_) is seen the nucellus, _n_; _m_ the micropyle. In the interior of the nucellus is seen an oval mass of cells, the endosperm, and at its top two archegonia, _c_. The ovule is turned in such a way that the micropyle points upwards, but usually it turns downward in the _Abietineæ_.] [Illustration: FIG. 252.--The apex of the nucellus (_n_) of an ovule of _Abies_: _l_ long-shaped cells which guide the pollen-tube; _s_ the wall of the macrospore (embryo-sac); _h_ the neck-cells of the archegonium; _k_ the ventral canal-cell; and _c_ the central cell (oosphere). The archegonia of the Cryptogams should be compared with this (see pages 181, 208, 216).]