CHAPTER IX ' 



DIVISION IV. SPERMATOPHYTA OR SEED PLANTS 



118. General Characters. This is the largest group of the 

 vegetable kingdom and includes all our common trees, shrubs 

 and herbs. The relationship of these plants is very imperfectly 

 understood and the various classes, orders, etc., into which they 

 are divided are in part artificial and do not therefore represent 

 completely the alliances of the groups. You have noticed that 

 the rporophyte is the most important feature in the life history 

 of the fern and that the gametophyte, subordinate from the start, 

 becomes very inconspicuous in some of the forms. This in- 

 equality of the two generations is more noticeable in the seed 

 plants where a progressive series of variations result in a more and 

 more complex external and internal differentiation of the various 

 organs of the sporophyte and in a steady reduction of the gameto- 

 phyte. Heterospory that appeared in several groups of the Pteri- 

 dophyta, becomes a constant characteristic of the seed plants. 

 The spores originate much as in the preceding group but the 

 megaspore is nourished and permanently retained in the sporan- 

 gium where it not only forms the female gametophyte (see Selagi- 

 nella) but also develops the young sporophyte. At this point, 

 the most characteristic feature of the Spermatophyta is seen. 

 The young sporophyte or embryo usually ceases to grow while 

 still in the gametophyte and passes into a resting or dormant con- 

 dition and the sporangium which has become variously modified 

 for the purpose of protecting the embryo is discharged from the 

 sporophyte. This sporangium containing the embryo is called 

 the seed. Contrast with these features two important differences 

 that appeared in Selaginella. First: The spores of these ferns 

 are retained and nourished in the sporangia only during the 

 early development of the gametophytes. Second: The young 

 sporophyte of Selaginella it may also be called the embryo 

 does not normally pass into a resting sjate, but steadily grows on 



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