[Chap. L THE SEED PLANTS 687 



of the plumule, many cotyledons, and the hypocotyl, at the end of which 

 is the root primodium. The seed thus consists of the embryo, female 

 gametophvte, perisperm (nucellus), and integument. 



In late autumn or early winter, the tissues of the ovulate cone become 

 dry, the sporophylls curl outward, and the seeds are liberated. The 

 seed has a thin membranous wing derived from the upper part of the 

 scale on which it develops. 



Summary. The principal steps in the life history of the pine are: (1) 

 the formation of microspores and pollen in microsporangia of the stami- 

 nate cone; (2) the development of two sperms in the pollen grain; (3) 

 dehiscence of the microsporangia and liberation of the pollen grains; 

 (4) the formation of megaspores in megasporangia (ovules) of the 

 ovulate cone; (5) development of the female gametophyte from the 

 megaspore within the ovule; (6) formation of archegonia, each con- 

 taining an egg cell; (7) pollination and growth of the pollen tube into 

 the archegonium; (8) union of a sperm with the egg, resulting in a 

 zygote; (9) development of embryo from the zvgote, further growth of 

 the "endosperm," and formation of seed coats from ovule coats. 



The pine tree is a sporophyte. The male gametophyte is a simple 

 structure composed of the cells within the pollen grain. The female 

 gametophyte is a small mass of tissue within the nucellus of the ovule. 

 The gametophytes of the pine, in sharp contrast to those of liverworts, 

 mosses, and some ferns, are never green and are parasitic upon the 

 sporophyte. Flagellate sperms occur in C)cads, but not in the other 

 gymnosperms (Fig. 329). 



Classification of gymnosperms. The living gymnosperms include four 

 orders: 



Cycadales ( cycads ) : Zamia, Dioon, Macrozamia 

 Ginkgoales: Ginkgo (maidenhair tree) 

 Gnetales: Titmboa, Gnetum, Ephedra 



Coniferales: Pintis, Larix, Taxus, Picea, Thuja, Sequoia, Juniperus, 

 Podocarpus, Auracaria 



The Angiosperms: Flowering Plants 



About 150,000 flowering plants have been described. This is about 

 half of all the plants known. They have the greatest diversity of vege- 

 tative and reproductive structures. They may be found in all land and 

 shallow water habitats, except the permanent snow and ice fields and 



