CHAPTER IX 



DIVISION IV. SPERMATOPHYTA OR SEED PLANT 



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 sporophyte is the most important feature in the life history 

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

 becomes very inconspicuous in the higher forms. This inequality 

 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 ferns, 

 becomes a constant characteristic of the seed plants. The spores 

 originate much as in the ferns but the megaspore is nourished 

 and permanently retained in the sporangium where it not only 

 forms the female gametophyte (see Selaginella) but also develops 

 the young sporophyte. At this point, the most characteristic 

 feature of the Spermatophyta is seen. The young sporophyte 

 or embryo ceases to grow while still in the gametophyte and 

 passes into a resting or dormant condition and the sporangium 

 which has become variously modified for the purpose of pro- 

 tecting 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 Selagi- 

 nella — it may also be called the embryo — does not normally pass 

 into a resting state, hut steadily grows on into the mature sporo- 



320 



