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VEGETABLE PHYSIOLOGY 



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 Spermophytes or flowering plants, 

 and now 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 microstores and megaspores respec- 

 tively. In the Pteridophytes the megaspores, when formed, 

 differ from the microspores chiefly in size ; in the Spermo- 

 phytes they are never liberated from the sporangium and 



have consequently thin and 

 delicate walls. 



The phenomenon of 

 heterospory involves the 

 production of two gameto- 

 phytes to one sporophyte, 

 as each of the spores pro- 

 duces its appropriate pro- 

 thallium. 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 gametophytes, the latter occurring 

 synchronously. 



The male gametes are free-swimming antherozoids in all 



Pteridophytes and are developed in antheridia of varying 



structure. The females are oospheres, produced in archegonia. 



The gradual appearance or development of the seed can 



FIG. 175. GERMINATION OF A MASS CF 

 MICROSPORES 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. 



