i66 



THE POPULAR SCIENCE MONTHLY. 



of them bearing the small males pores, and the third, a, the large fe- 

 male spores. 



In the next higher group, as arranged by botanists, we find the 



club-mosses ; these are common plants with 

 trailing or upright very leafy stems. Fig. 

 13, A, shows the tip of a spore -bearing 

 branch, natural size, and B a longitudinal 

 section much enlarged. The large spores 

 are borne in sporangia on one side, while 

 the small ones are on the left. The differ- 

 entiation has now reached the j^lace where 

 there is a definite arrangement of the spo- 

 rangia on the plant bearing them. In the 

 development of the male spores the cells in 

 which the antherozoids form are not produced directly from the spore- 

 contents. This is a valuable link in the chain of relationship which 

 binds this group with higher plants — in fact, helps to bridge what gulf 



there may have been thought existing 

 between the flowerless and flowering 

 plants. The ripe male spore-contents 

 are changed into a few cells, one of 

 which remains sterile and is consid- 

 ered the prothallus, while from the 

 other cells — which taken as a whole 

 constitute the antheridium — the cells 

 which afterward bear the antherozoids 

 are formed. In the genus Selaginella 

 the female spore produces a small pro- 

 thallus, as shown at 1, Fig. 14. The 

 portion above d d, in this cross-section 

 of the spore, is the prothallium, and at 

 e e are two embryo jjlants. At 2 is a 

 young archegonium not opened ; 3 

 shows one further advanced, with the 

 fertilized germ-cell divided. A is a 

 male spore, showing the cell-division ; 

 D is a later state of the same, with the 

 large antheridium filled with sperm- 

 cells. The rudimentary prothallus is 

 at V. The female is still more simple 

 in Isoetes, shown in Fig. 15 : 1 is the 

 longitudinal section of the female 

 spore, with an archegonium, ar, at 

 the top ; 2 shows the early differen- 

 tiation of cells into archegonia, a r, a r, with their germ-cells, [/ ff / 

 3, 4, and 5 show successive stages in the development of the germ-cell. 



Fig. 13. 



