. An introduction to vegetable physiology. Plant physiology. REPRODUCTION 429 them, and which leads to the production of the structure known as the seed, the latter being a special body produced by all members of the group of Phanerogams or flowering plants, and marking them off clearly from all below them. The phenomenon in question is known as heterospory. Plants which exhibit it bear two kinds of spore, which differ from each other mainly in their relative dimensions. Some are produced in large numbers in a sporangium and have usually the structure which has already been described. Others a
. An introduction to vegetable physiology. Plant physiology. REPRODUCTION 429 them, and which leads to the production of the structure known as the seed, the latter being a special body produced by all members of the group of Phanerogams or flowering plants, and marking them off clearly from all below them. The phenomenon in question is known as heterospory. Plants which exhibit it bear two kinds of spore, which differ from each other mainly in their relative dimensions. Some are produced in large numbers in a sporangium and have usually the structure which has already been described. Others are much larger than these and are developed either singly or in small numbers, usually four in a sporangium. They are spoken of as microspores and megaspores respec- tively. In the Pteridophytes the megaspores when formed differ from the microspores chiefly in size; in the Phanero- gams they are never liberated from the sporangium and have consequently thin and delicate walls. The phenomenon of heterospory involves the production of two gametophytes to one sporophyte, as each of the spores produces its appropriate prothallium. The gameto- phyte arising from the microspore gives rise only to male gametes, that from the megaspore only to female ones. Such plants show in their life cycle, therefore, three forms, one sporophyte and two game- tophytes, the latter occur- ring synchronously. The male gametes are FlG. free swimming anthero- zoids in all Pteridophytes and are developed in an- theridia of varying struc- ture. The females are oospheres, produced in archegonia. The gradual appearance or development of the seed can. ION OP A MASS OF Miceospobes of Salvinia. (After Sachs.) 1, The mass protruding tubular prothalli from different spores; 2, a prothallus more highly magnified, showing an antheridium, a; 3, antherozoids in mother-cells; 4, ruptured Please note that these images are extracted from scanned page images that may have been digitally enhanc
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