194 THE EVOLUTION OF PLANTS 



two kinds of stem above ground, one fertile, 

 bearing the cone, and the other vegetative. The 

 former is short, unbranched, pale in colour, and 

 short-lived; the latter is tall, freely branched, 

 green, and more lasting. In other species, how- 

 ever, such as the Smooth Horsetail (E. limosum), 

 common in ponds, there is no such distinction, 

 and the cones are borne on the ordinary green 

 stems. In all cases there are also branched 

 underground stems, penetrating the soil to a 

 great depth, and making the plants most diffi- 

 cult to get rid of when once established. In fact, 

 the Horsetails, though so small a remnant of an 

 ancient family, are still a vigorous race and well 

 able to hold their own; in lakes in the Hebrides, 

 as already mentioned, they are said to be spread- 

 ing rapidly at the present day, and displacing the 

 aquatic Flowering Plants. 



The leaves of the Horsetails are peculiar and 

 much reduced, taking little part in the nutritive 

 work (assimilation of carbon from the carbonic 

 acid of the air) which is ordinarily assigned to 

 leaves. Each ring or whorl of leaves forms a con- 

 tinuous sheath, fitting round the stem; the little 

 teeth projecting from the top of the sheath alone 

 represent the free parts of the leaves. 



The usual work of the leaf is in the Horsetails 

 taken over by the finer branches, which are 

 green, or even by the main stem itself. The 



