22 METASPERMAE OF THE MINNESOTA VALLEY 



Examples of the Metaspermae may be selected from the great 

 mass of plants which contain their seeds in a closed •■ ovary." 

 better named carpellum. Such plants range in structure from 

 such lower forms as Salix and Typlia to the highly developed 

 Orchidaceae, Urribelliferae and GomposUae, including such plants 

 as Listera, Myrrhis and Hieracium. 



A more definite characterisation of the Metaspermae may be 

 added to the diagnostic limitation given above. 



Characters of the Metaspermae. The Metaspermae, other 

 wise called Angiospermae, are those Sporophyta which produce 

 constantly polymorphic species-forms, consisting of always 

 bisexual, vegetatively degenerate, parasitic gametophytic 

 plants and always (a) bivalent sporophytic plants, one of which 

 is produced from a close-fertilised Qgg and develops an endo- 

 sperm of the seed, while the other is produced from a cross- 

 fertilised egg and develops the embryo of the seed. — which 

 latter, in turn, upon the germination of the seed, normally 

 resumes development and matures into a structure of high 

 vegetative specialisation from which are ultimately developed, 

 either one or both sizes of spores, and from these the sexual 

 plants are respectively produced. The smaller spores or 

 pollen-grains are produced numerously in sj^ecial spore-cases 

 (sporangia), aggregated upon specially modified foliar or 

 axillary structures called stamens. The larger spores are 

 produced severally or, more commonly, singly, in a special 

 sporangium (nucellus of ovule) surrounded with indusial mem- 

 branes (ovular integuments) and the sorus (ovule) thus formed 

 is borne in a closed foliar or axillary structure called a pistil. Of 

 this closed pouch the actual seed-bearing cavity (ovary or 

 carpelhim) ripens into the fruit. which is always at first a closed 

 structure The seed is a ripened sorus commonly detachable 

 from the structure upon which it was produced. It contains 

 within the modified indusial walls (seed-coats) two sporophytic 

 plants of different valency. One, produced from an Qgg fertil- 

 ised by the sperm nucleus from the pollen-tube, is alone termed 

 the embryo The other, produced from a close -fertilised egg, is 

 termed the endosperm, and is consumed by the embryo either 

 during the ripening processes of the seed or during the germi- 

 nating processes of the same. 



It will be interesting to see how the Archispermae or lower 

 seed-plants ( Gymnospermae) differ from the Metaspermae. The 

 fact that seeds are such distinct, easily defined bodies, in com 



(a). Except in some Orchidaceae? 



