DEVELOPMENT OF PLANTS 



361 



only are the male and female gametophytes better provided for 

 than in Selaginella, owing to their complete parasitism on the 

 well developed sporophytes, but the formation of the young 

 sporophyte or embryo is also ensured by the abundance of food 

 placed at its disposal. The most important advantage, however, 

 possessed by the seed plant appears in the modification of the 

 integument which renders it impervious to gases and fluids. 

 This change so effectually seals up the embryo as to protect it 

 for years in some cases against conditions unfavorable for growth. 

 The modification of the integument was doubtless the chief 

 factor that led to the seed habit, since it cuts off the supply of 



Fig. 249. Renewal of growth of the sporophyte or embryo in the seed. 

 7, section of the seed, showing the base of the cotyledons extending from the 

 seed, thus pushing out the stem and root, the latter organ (shown in part) 

 curving down into the ground. The free ends of the cotyledons are enlarging 

 as they absorb the food stored in the gametophyte. 6, seedling six months 

 old with first normal leaf. — After Sachs. 



oxygen and fluids from the sporophyte or embryo and thus 

 stops its growth. When conditions are favorable for growth 

 (see page 129), the integument becomes permeable to fluids and 

 gases and the embryo continues its development. The cotyle- 

 dons remain in the seed to absorb the food stored in the gameto- 

 phyte, but their basal portions elongate, pushing out the root 

 which soon becomes established in the soil, while the stem which 

 is carried out with the root extends up into the air, and de- 

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