BOTANY 



ment takes place under the protection afforded 

 within the body of the mother. 



As Professor Oliver has said, " With the evolu- 

 tion of the seed, the plant rose at a bound to a 

 higher plane, and this structure in its perfected 

 form has become the very focus of the plant's 

 existence." Botanists recognise that the essential 

 feature in the evolution of a seed was the retention 

 of the megaspore within the sporangium after 

 the number of functional megaspores had been 

 reduced to one. An approach to this condition 

 is to be seen in the living Lycopod genus Selagi- 

 nella, where the number of megaspores is reduced 

 to a single tetrad of four, which germinate within 

 the megasporangium. In the Palaeozoic Lycopod 

 Miadesmia this process went a stage farther ; the 

 single megaspore is retained in its sporangium, 

 which is in turn protected by an integumental 

 covering leaving only the apex exposed at the 

 micropyle. In a related form, Lepidocarpon, a 

 seed of similar type is found, with one functional 

 megaspore and three abortive ones, and Dr Scott 

 has shown that as this seed develops an integument 

 grows up as a new structure and surrounds the 

 sporangium. 



From the point of view of protection to the 

 developing embryo, the condition in Selaginella, 

 in which the female gametophyte develops within 

 the megaspore, only rupturing its wall when nearing 



155 



