354 



BOTANY: PRINCIPLES AND PROBLEMS 



directly to the archegonia. After fertilization there are a few 

 divisions of the egg within the archegonium itself, and the young 

 proembryo thus formed is then carried deeply into the tissues of 

 the embryo-sac by certain of its upper cells, which rapidly 

 elongate. In this position it develops into the mature embryo 

 of the seed. 



Fig. 225. Fig. 226. 



Fig. 225. — Longitudinal section (diagrammatic) of the ovulate or "female" 

 cone of pine. Attached to the base of each scale is seen an ovule, its micropyle 

 pointing inward. 



Fig. 226. — Fertilization in a conifer. Archegonium into which a pollen-tube 

 has just entered. One of the male nuclei may be seen uniting with the egg 

 nucleus. The other, left behind in the cytoplasm, will die. (From Sinnoll). 



Like cycads, the conifers are an ancient group and are promi- 

 nent in fossil floras since Mesozoic times. Although they include 

 only about 350 species today, they cannot well be called degenerate 

 for in many parts of the forested regions of both the north and the 

 south temperate zones they contribute to the vegetation such 

 notable trees as the pines, spruces, firs, larches, hemlocks, cedars, 

 cypresses, and many others. 



3. Gnetales. — Brief mention should be made of the most highly 

 specialized order of gymnosperms, the Gnetales, which consist of 

 three genera only; a tropical climber, a desert shrub, and an 



