. Foundations of botany. Fig. 215. —a Water-Fern{Salvinia). nil. Fig. 216 Twolndusiaof Salvinia. mi, microspores ; ma, macro-spores. THE EVOLUTIONARY HISTORY OE PLANTS 303 development the macrospore produces an endosperm whichis really a small cellular pro thallium, concealed in the ovule.The microspore contains vestiges of a minute prothallium. In the angiosperms the macrospore and its prothalliumare still less developed, and themicrospore, or pollen grain, haslost all traces of a prothalliumand is merely an antheridiumwhich contains two generativecells.^ These are most easilyseen in the poll
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. Foundations of botany. Fig. 215. —a Water-Fern{Salvinia). nil. Fig. 216 Twolndusiaof Salvinia. mi, microspores ; ma, macro-spores. THE EVOLUTIONARY HISTORY OE PLANTS 303 development the macrospore produces an endosperm whichis really a small cellular pro thallium, concealed in the ovule.The microspore contains vestiges of a minute prothallium. In the angiosperms the macrospore and its prothalliumare still less developed, and themicrospore, or pollen grain, haslost all traces of a prothalliumand is merely an antheridiumwhich contains two generativecells.^ These are most easilyseen in the pollen grain, butsometimes they are plainly visi-ble in the pollen tube (Fig. 164). Phanerogams are distinguishedfrom all other plants by theirpower of producing seeds, orenclosed macrosporangia, withembryos. 375. The Law of Biogenesisand the Relationships of the GreatGroups of Plants. — On summingup Sects. 372-374 it is evidentthat the sexual generation ingeneral occupies a less and lessimportant share in the life of theplant as one goes higher in the scale of