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TEXTBOOK OF BOTANY 



In the central part of the macrosporc sac, four macrospores 

 are formed (Fig. 73, C) ; they are much larger than micro- 

 spores. All the spores that we have studied hitherto were 

 produced in such a way that when they were ripe they could 

 escape from the parent plant. But the macrospores of the 



Fig. 73. — A, a macrospore leaf of the pine bearing two macrospore 

 sacs (ovules). 5, a lengthwise section of a macrospore leaf ; i, integument; 

 m, micropyle. C, part of a lengthwise section of an ovule on a larger scale, 

 showing the four macrospores. 



pine are so tightly imbedded in the tissues of the spore sac 

 that they cannot escape, and so if they are to germinate 

 they must do so inside the spore sac. This imprisoning of 

 the macrospores is one thing that distinguishes the seed 

 plants, and it leads to some very important consequences. 



149. Germination of the Macrospore. — Of the four 

 macrospores, only one germinates. It develops into a small, 

 rounded mass of tissue which, since it grows from the spore, 

 belongs to the sexual generation of the pine. This mass of 

 tissue is a female plant; but, because of the position of the 

 spore from which it has grown, this small plant remains inside 

 the macrospore sac (Fig. 74). At the end of the female 

 plant nearest the micropyle, archegones (one to nine in 

 different species) are formed. Each archegone consists of a 

 large egg and a very short canal that leads from the surface 

 of the female plant to the egg. 



