THE FERN'S PLACE IN NATURE. 51 



cases (Lemna, etc.), a well-developed leafy axis containing 

 highly differentiated tissues of every kind ; sexual reproduction 

 consisting of the union of pollen grains (male element) with the 

 embryo-sac (female element), resulting in the formation of an 

 embryo, which with its coverings constitutes the seed.* Con- 

 tains two well-marked classes. 



1. GYMNOSPERM^E. (Cone-bearing trees, Cycads, etc.) 



2. ANGIOSPERM^:. (All other seed-bearing plants.) 



1 38. It will thus be seen that the Ferns and their Allies 

 occupy a high place in the plant world just below the seed-pro- 

 ducing plants. This position they maintain not only from 

 complexity of structure and the character of their reproduction, 

 but also from their evident graded relation to some of the 

 lower forms of spermaphytes, including many now extinct. 



LITERATURE. 



In preparing this list of reference works on the various 

 groups of plants, a list recently published by Dr. Besseyt has 

 served as a basis. Numerous additions have been made, and 

 such comments have been introduced as will enable the student 

 to select intelligently the most desirable works. As the lower 

 groups of plants have been classed heretofore in the two great 

 divisions algce and fungi, we begin with general works on these 

 two groups, and then indicate special works and papers relating 

 to narrower groups, as they have been independently mono- 

 graphed. 



Algae. 



The algae are frequently grouped according to habitat. 

 The works relating to fresh-water forms are : 



WOLLE (F.). Fresh-water Algae of the United States. 2 

 vols. 8vo. Bethlehem, Pa., 1887. 



* It will be readily seen that this method is only a slight modification of 

 what appears in the development of the higher forms of Pteridophytes like 

 Selaginella, the microspores corresponding to the pollen and the macro- 

 spores to the embryo-sac. The prothallium, which in ferns is a marked fea- 

 ture of the sexual stage, becomes reduced in Selaginella, and disappears ex- 

 cept in rudiment in the seed plants. There are no sharp lines possible in 

 classification. 



t American Naturalist, XXI, 376-379 (April, 1887). 



