IX] EQUISETUM. 245 



mose. The stem is divided into comparatively long internodes 

 separated by the leaf-bearing nodes, and the branches arise in 

 the leaf-axils at the nodes. The fertile leaves or sporophylls 

 differ from the sterile leaves, and usually occur in definite 

 ^'ggi'egations or strobili containing spores of one kind {iso- 

 sporous). In the single living genus Equisetum, the outer 

 coat of the mature spore forms two hygroscopically sensitive 

 filamentous structures or elaters. On the germination of the 

 spore the gametophyte is developed in the form of a small lobed 

 prothallium 1 — 2 cm. in length. In most cases there are distinct 

 male and female prothallia. 



The genus Equisetum L., the common Horse-tail, is the sole 

 living representative of this Family. It occurs as a common 

 native plant in Britain, and has a wide geographical distribution. 

 Species of Equisetum are abundant in the temperate zones 

 of both hemispheres, and occur in arctic as well as tropical 

 latitudes. Wallace^ speaks of Horse-tails, "very like our own 

 species," growing at a height of 5000 feet on the Pangerango 

 mountain in Java. In favourable situations the large British 

 Horse-tail, Equisetum maximum Lam. (= E. Telmateia Erhb.), 

 occasionally reaches a height of about six feet, and growing in 

 thick clusters forms miniature forests of trees with slender 

 erect stems and regular circles of long and thin branches. 

 A tropical species, Equisetum giganteum Linn." living in the 

 marshes of Mexico and Cuba'', and extending southward to 

 Buenos Ayres and Chili, reaches a height of twenty to forty 

 feet, but the stem always remains slender, and does not exceed 

 an inch in diameter. Groves of such tall slender plants on 

 the eastern slopes of the Andes' suggest to the palaeobotanist 

 an enfeebled forest-growth recalling the arborescent Calamites 

 of a Palaeozoic vegetation. The twenty-five existing species 

 of Equisetum are remnants of various generic types of former 

 epochs, and possess a special interest from the point of view 

 of the geological history of plants. A brief description of the 



1 Wallace (86) p. 117. 



2 Baker (87) p. 4. Hooker, W. J. (61) PI. lxxiv. Vide also Milde (67) for 

 figures of Equisetum. . 



^ Seaman (65). 



