64 PLANTS AND MAN 



The significance of the innovations found in the Httle club 

 moss Hes in the resemblance of its microspores to the pollen 

 grains of the seed plants, which are merely highly specialized 

 microspores. The ovules found in the pistil of a flower are like- 

 wise specialized sporangia producing megaspores. Thus when 

 pollination takes place, there is a transfer of the microspores 

 with their minute male prothalli to the megaspores containing 

 their microscopic female plants. The details of this process, with 

 the structures resulting, will be made clear as we consider the 

 reproductive life history of a Gymnosperm such as the pine tree. 



The Reproductive Cycle of a Cone-bearing Seed Plant 



The PINE can be considered typical of the whole group of 

 primitive seed plants (Gymnosperms) in which alternation of 

 generations results in the production of two kinds of spores as in 

 the little club moss, in an independent green sporophyte as in all 

 the ferns, and in a parasitic gametophyte as in the little club moss. 

 Improvements center about the special reproductive organs 

 which produce the spores, known as cones; the novel method by 

 which dependence upon motile sperm is eliminated, resulting 

 in the formation of a pollen tube; and the protective and nutri- 

 tive tissues which surround the embryo sporophyte, resulting in 

 the seed. 



Cones are familiar structures found on many Gymnosperm 

 trees such as the spruces, firs, cedars and pines. These cones are 

 the reproductive organs produced by the sporophyte generation, 

 which is the tree itself. Taking a pine as an example, we find 

 that the sporophyte develops two kinds of cones (fig. 38). One of 

 these is the large cone composed of overlapping scales, amidst 

 which the seeds appear. Seeds are always the product of sexual 

 reproduction, and therefore develop from fertilized eggs which 

 appear only on the female prothalli; since female prothalli 

 (gametophytes) are produced only within the megaspores, such 

 seed-bearing or "female" cones are in reality cones bearing large 

 spores. The other kind of cone is much smaller and less con- 

 spicuous; oval in outline, they grow in clusters near the tips of 

 the branches, usually higher up on the tree than the seed-bearing 

 cones. These are sometimes called "male" cones because they 



