344 NATURE STUDY AND AGRICULTURE 



sexual plant. It is no wonder that the early students of 

 ferns could find no sex organs, for the plants they ex- 

 amined were sporophytes. 



Certain plants known as "club mosses" and "ground 

 pines" belong to the fern group and are better than the 

 common ferns to illustrate the other points of advancing 

 differentiation which appear in this group. They are 

 often used for Christmas greens and are found in abundance 

 in the pine woods of the North. Their long prostrate 

 stems are thickly covered with narrow, pointed, green 

 leaves, giving them quite a tufted or bushy appearance, and 

 explaining the name ' ' ground pine." Now the sporophyte 

 in all members of the fern group bears on its leaves special 

 spore-producing organs called "sporangia." Clusters of 

 these sporangia forming brown dots or patches are matters 

 of common observation on the under sides of fern leaves. 

 At first, in the evolution of ferns, all the leaves bore sporangia, 

 but presently there appeared leaves of two kinds, one being 

 an ordinary foliage leaf doing exclusively the work of food 

 making and bearing no sporangia at all, the other doing no 

 ordinary foliage work and being devoted exclusively to the 

 bearing of sporangia. The latter kind of leaves are called 

 sporophylls or spore leaves. Sometimes they do some 

 ordinary leaf work as well as the work of sporangia pro- 

 ducing. Now in the club mosses we find these sporo- 

 phylls grouped together in a club-shaped tuft at the end of 

 the branches, and this definite, conelike cluster of sporo- 

 phylls is called a strobilus. The strobilus is a very impor- 

 tant organ to understand, because it is the forerunner of 

 that very familiar organ the flower. It is fair to say that 

 the flower cannot be really understood unless we under- 



