i6o THE POPULAR SCIENCE MONTHLY, 



tached from the parent-plant. The offspring may, or may not, be like 

 the parent. Plants which result from similar cells developed in the 

 same manner belong to the same generation. If the original repro- 

 ductive cells grow into plants without any union with other cells, these 

 plants are asexually produced. If the union of the contents of two or 

 more cells is necessary for further development, there results a sexual 

 product. 



It is the purpose of this paper to trace the relative size of the two 

 generations above described, in a number of the higher orders of plants. 

 In doing this, the fact of the alternation will be developed. The series 

 of orders will begin with the humble Hepaticoe, and end with the most 

 highly developed of flowering plants. The Hepaticce, or liverworts, 

 are small, flowerless plants of very simple structure, which grow for 

 the most part in moist places upon the bark of trees, surface of long- 

 exposed rocks, earth, etc. One of the leading genera is Marchantiay 

 species of which abound on the earth of flower-pots in greenhouses 

 and elsewhere. The leaf -like expansion or thallus is the sexual gen- 

 eration, and bears the male and female organs in depressions of the 

 surface. The male parts, called antheridia, produce spermatozoids, 

 which are spiral, slender bodies, provided with two motile hairs or 

 cilia, as locomotive appendages. The female organs (archegonia) are 

 at first single cells, which by division form flask-like structures, the 

 lower cell of which is the female germ-cell. When this germ is ferti- 

 lized by the antherozoids, which enter at the neck of the " flask," it 

 undergoes a development, varying somewhat in the different orders, 

 but essentially a sporangium or spore-case is produced, in which are 

 very many spores and slender spiral threads 

 arranged in rows. This sporangium is the W& 



second and asexual generation of the liver- ^\S 



wort. The complexity of the structure of 



Fig. 1. 



the first or leaf -like generation and of the sexual organs and sporangia 

 increases in the hepaticae group in passing from the lower to the higher 

 forms. In the highest group there are stems with leaves arranged in 



