MARSH AND SMOOTH HORSETAILS 97 



marshes, and ditches, so that on the whole it prefers a wetter 

 habitat than the field and wood horsetails, but it also occurs 

 in gravelly places and (at least in the south of England) on dry 

 exposed hills, especially on chalk downs near the sea. The 

 fertile shoots, which appear in spring and are short-lived, are 

 shorter but stouter than the barren shoots, and have large over- 

 lapping brown sheaths, each with thirty to forty long pointed 

 teeth. Sometimes the later shoots bear a small cone at the 

 top above the branches. The Marsh and Smooth Horsetails 

 (E. palustre, E. limosum} are confined to marshes and ponds, 

 and often grow in dense masses. Both kinds are very common; 

 in E. palustre the stem is fluted and feels rough, in E. limosum 

 it is quite smooth and even. It will be noticed that, on the 

 whole, the more distinctly water-loving horsetails are less 

 branched, and have fewer ridges and furrows (or none at all) 

 than those which grow in drier places. The marsh kinds grow 

 in crowded masses, hence the few and short branches, as com- 

 pared with the less gregarious roadside and woodland kinds 

 (e.g. field and wood horsetails). The stomates on the stem 

 are found in the furrows, hence the stems are more deeply fur- 

 rowed, and the furrows more numerous in the kinds growing 

 in dry places (e.g., field horsetail) than in the water-loving kinds. 

 The horsetails show a curious mixture of adaptations. The 

 leaves are reduced to scales, and the work ordinarily carried on 

 by leaves is done by the green stem and branches, while the 

 area from which water is lost (by transpiration) is further reduced 

 by the restriction of the stomates to the parts of the skin (epi- 

 dermis), which line the grooves on the" stem. In correspondence 

 with this the water-conducting bundles, or veins, are feebly 

 developed. Adaptations of this kind, for preventing excessive 

 loss of moisture, are characteristic of plants growing in dry 

 places. On the other hand, the horsetails also show a great 

 development of air-spaces, characteristic of water-inhabiting 

 plants. This combination of characters has been explained as 

 follows. The horsetails live chiefly in mud or stagnant water, 

 so that the roots are badly supplied with air, therefore fresh air 

 must be taken in by the upper parts of the plant, and conveyed 

 to the creeping stem and the roots along the air-passages. At 

 VOL. iv. 7 



