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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 Phane- 

 rogams 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 gradual appearance 

 or development of the seed 

 can be examined by study- 

 ing a series of forms. The 

 earliest indication of it 

 which we can find is ex- 

 hibited by the Hydropteridece, of which Salvinia is a 

 characteristic type. Salvinia is a heterosporous form, each 



FIG. 177. GERMINATION OF A MASS OF 

 MICROSPOBES 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 antheridium. 



