482 



PART III. THE CLASSIFICATION OF PLANTS. 



microsporangiftte flower is a cone, consisting of an elongated axis bearing 

 microsporophylls (Fig. 304), which are generally somewhat peltate in form. 

 Each microsporophyll bears two or more microsporangia on its under (dorsal) 

 surface. The macrosporangiate flower is also a cone in certain cases (Pinoideae, 

 Fig. 305), in which case the macrosporophylls bear the macrosporangia ; in 

 other cases there is a less perfect cone, or none at all (Taxoideae, see Fig. 308), 

 the macrosporophylls are either rudimentary or absent, and the macrosporangia 

 are generally borne on the axis. 



In some genera (e.g. Pinus, Juniperus) the seed takes two years to ripen ; in 

 the first year, pollination takes place, and the pollen-tube begins to grow 

 through the tissue of the nucellus ; in the second year, after a period of rest, 



FIG. 304. Pinus montana (Pumilid). A Longitudinal section of a microsporangiate flower 

 (x 10). B Longitudinal section of a microsporophyll, showing the cavity of one pollen-sac 

 (x 20). C Transverse section of a microsporophyll, showing the cavities of both pollen- 

 sacs. D Germinating two-celled miorospore of Pinus sylvestris, showing the expansions of 

 the exine (x 400). (After Strasburger.) 



the pollen-tube completes its growth, reaches the archegonium, and fertilises 

 the oosphere ; as a consequence, the embryo is developed, and the ovule is 

 changed into a seed. 



In spite of the fact that so many of the Conifer are polyembryonic (see 

 p. 471), and that each ovule contains several archegonia, the ripe seed contains 

 only a single embryo, though occasionally two are found (e.g. Ginkgo). The 

 embryo has two, or more, cotyledons, which are epigean, except in Araucaria, 

 sect. Colymbea, and in Ginkgo, where they are hypogean. 



The order, which includes 34 genera and about 350 species, may be natur- 



