APPENDIX 239 



of the rigid tissue and fibro-vascular bundles. Fern spores may be 

 germinated in sweetened water. 



21. The pine. The pine is one of the best types of seed plants, 

 easily available, to make clear the relationships between pteridophytes 

 and spermatophytes. The general homologies between the staminate 

 cone and the cones of the club mosses and horsetails are easily under- 

 stood, as well as the homologies between the pollen grain with its 

 inclosed male gametophyte and the microspores of pteridophytes with 

 their male prothallia. The female gametophyte of the pine, also, is 

 readily compared with the reduced female prothallia of heterosporous 

 pteridophytes. For these reasons the life history of the pine is more 

 easily understood with reference to the life histories of pteridophytes 

 than is the life history of an angiosperm (such as the lily) where the 

 gametophytes are much more reduced in structure. 



The pine is also excellent for the study of the tissues of a tree with 

 annual growth from a cambium ring. The structure of pine wood is one 

 of the best exercises in the interpretation of cell structure, and the study 

 outlined in Sec. 138, C, 5, is often given as a laboratory problem. 



In a course outlining the evolution of life histories in plants the pine 

 is as important a type as Selaginella, the fern, or the moss, besides having 

 in itself a remarkably interesting morphology in relation to peculiar life 

 habits. 



22. The lily. There are perhaps no types more convenient than 

 those of the lily family for the study of the gametophytes of the angio- 

 sperms. It is, however, not easy to present the life history of angio- 

 sperms in full from the study of a single type in a general course. It is 

 probably better to use various forms which may be especially favorable 

 for particular phases ; as, for example, the lily for the development of 

 the pollen, but the elder for the male gametophyte ; the lily for the 

 development of the embryo sac together with fertilization, double 

 fertilization, and the origin of the endosperm, but the shepherd's purse 

 for the development of the ovule and embryo. 



23. The shepherd's purse. This plant would be almost perfect for a 

 complete study of an angiosperm life history were it not for the small 

 size of the flowers, anthers, and pistil, and in consequence the minute- 

 ness of the gametophytes. However, the shepherd's purse is one of the 

 best types for the study of floral development (notwithstanding certain 

 irregularities) and the development of the ovule and embryo. 



