SIGNIFICANCE OF THE SEED IN EVOLUTION 385 



The progress to the development of the ovule depends 

 on the retention of the megaspore within the megasporan- 

 gium and the appearance of some means other than 

 their own locomotion in water of bringing the male 

 gametes to the female. We do not know all the steps 

 in the evolutionary process, because we have not 

 discovered the fossil forms which were the actual 

 ancestors of our modern seed plants. But the essential 

 new features are two, first the appearance of a wall of 

 cells to the megasporangium (nucellus of the ovule) and 

 of one or two coats of cells covering the nucellus, and 

 secondly of the germination of the microspore (pollen 

 grain) to form a germ tube (pollen tube) which can 

 grow to the neighbourhood of the egg, carrying the male 

 gametes with it. A further feature is the closure of the 

 leaf (carpel) round the megasporangia (ovules). This is 

 not found in the most primitive existing seed plants, such 

 as the pines and firs, which are called " Gymnosperms," I 

 because their ovules are not enclosed in carpels. 



The pollen grains are brought to the ovule in Gym- 

 nosperms, and to a special receptive organ of the carpel 

 (stigma) in most flowering plants, by the wind or by 

 insects, and the perfection of the method of their 

 transport has undoubtedly increased the chances of 

 fertilisation in a great variety of species. From the 

 stigma to the ovule the growth of the pollen tube and 

 the life of the male gametes it encloses are protected by 

 the fact that it takes place inside the tissue of the carpel. 

 The female gamete is well protected by nucellus, ovule 

 coats and carpel wall. All these arrangements enable 

 the process of fertilisation to be carried out in flowers 

 exposed to dry air in which exposed gametes would 

 at once be killed. 





1 Greek yvfivog, naked, and (rnfyfta, seed. 

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