Biological Papers. 



343 



which must germinate and produce another organism whose cells 

 have half the normal number of chromosomes, and which finally 

 produces eggs and sperms in peculiar organs. Plants which pro- 

 duce spores of equal size are isosporous, and plants which produce 

 spores of unequal size are heterosporous, the large spores being 

 named megaspores and tlie small spores microspores. The liver- 

 worts, mosses and common ferns are isosporous; and marsilia, 

 selaginella, the pines and spruces, and all our common flowering 

 plants produce spores of different sizes and are therefore hetero- 

 sporous. 



CONJUGATION. 



I to L. — On the conjugation of one of the sperm-cells with the 

 egg-cell, complete fusion of the cytoplasm and nucleoplasm takes 

 place, but the chromosomes do not unite but continue distinct 

 during myriads of succeeding cell divisions till the next period of 

 maturation. The three polar bodies of the egg-cell disappear as 

 does also its centrosome, the centrosome of the sperm-cell officiating 



'OTiju g^nii^^ 



--A-- 3 



in the cell divisions, It will be noted in K that the fertilized egg 

 has eight chromosomes, and that finally these unite to form two 

 spirems, shown in L, at the beginning of the next maturation after 

 myriads of cell divisions. The process of conjugation of eggs and 

 sperms is the same in plants and animals. 



PLANT FERTILIZATION AND EMBRYOLOGY. 



M to Q. — It has long been known that the spore-containing or- 

 gans of plants are usually modified leaves. Gray long ago taught 

 that the simple pistil is a leaf so modified that it forms a closed 

 chamber or cell in which the ovules (megasporangia) are developed 

 along its united margins. We know that the embryo-sacs of the 

 ovules are megaspores, that the ovules are megasporangia, and that 

 the pistils are megasporophylls. In like manner the stamens, us- 

 ually modified leaves, are now called microsporophylls; the anthers, 

 microsporangia, and the pollen grains, microspores. In diagram 

 J/, 1 is the nucellus containing the embryo-sac, 2; and 3 is a pollen 



