^44 Minnesota Plant Life. 



leaves, especially toward the tips of branches — therefore raised 

 higher from the ground — devoted themselves particularly to 

 spore-manufacture, leaving the starch-making to lower leaves 

 on the stem. Naturally it was more important for the plant 

 to use, for spore-production, those leaves farthest from the 

 ground, because if produced at a height the spores could be 

 distributed over a wider area. 



When the habit of making two kinds of spores became fixed 

 among the distant ancestors of modern seed-bearing plants, the 

 clusters of spore-bearing leaves or cones came, in the pines and 

 their allies, to be specialized, so that one cone devoted itself 

 to the manufacture of the small-spores while another produced 

 only large-spores in the little cases on its leaves. Thus there 

 originated the very different pollen-bearing and seed-bearing 

 cones, which may be observed in such plants as the white pine, 

 tamaracks and junipers. Probably, however, in some varieties 

 there was not this separation, upon their special axes, of the 

 two sorts of leaves; but one sort arose toward the tip of the 

 axis, while the other appeared lower down, so that })ii.vcd cones, 

 with carpels or large-spore-bearing leaves toward the tip, and 

 stamens or small-spore-bearing leaves toward the base of the 

 axis, originated. After the seed-habit had become fixed, it 

 is apparent that the same advantage which club-mosses de- 

 rived from having their spore-producing leaves highest on the 

 stem, would now be derived from having the seed-rudiment- 

 producing leaves, or carpels, highest on the stem. The advan- 

 tage of height above the substratum which was manifest in the 

 distribution of spores would be retained as favorable to the dis- 

 tribution of seeds. The primitive higher flowering jilants no 

 doubt had their seeds distributed by the wind, and it was, 

 therefore, important, from the plant's point of view, that the 

 carpels should stand higher in the flower than the stamens. 

 This will account for the central position of the pistil and the 

 peripheral position of the stamens. 



The crowding together of the leaves of llio tlower, which 

 became i)ossible when they abandonetl their starch-making 

 functions, may be seen foreshadowed even in the cones of the 

 club-mosses. It becomes still more marked in the pines, while 

 in flowering-plants the great majority have the cone or flower- 



