458 BOTANY PART n 



terminate in a single cone (Fig. 420). Resembling in their mode of life a parasite 

 upon the rhizome, they are otherwise distinguished from the vegetative haulms by 

 their lack of chlorophyll and their light yellow colour. 



Equisetum giganteum, growing in South America, is the tallest species of 

 the genus ; its branched haulms, supported by neighbouring plants, attain a 

 height of over twelve metres, and are about two cm. in diameter. 



The spores are all of one kind, and on germination give rise to thalloid 

 ruuniAM.iA, which are generally dicecious (B'ig. 421). The female prothallia are 

 larger than the male, and, branching profusely, are prolonged into erect, ruffled 

 lobes at whose base the archegonia are produced. In structure the archegonia 

 resemble those of the Ferns, but the upper cells of the four longitudinal rows of 

 cells constituting the n<;ck arc more elongated and, on opening, curve strongly 

 outwards. The first leaves of the embryo are arranged in a whorl and encircle the 

 apex of the stem. The growth of the embryo is effected by the division of a three- 

 sided apical cell (Figs. 164, 165). 



The outer epidermal walls of the stem are more or less strongly impregnated 

 with silica. In Equisetum hiemale, and to a less degree in Equisetum arvense, the 

 silicification of the external walls is carried to such an extent that they are used 

 for scouring metal utensils and for polishing wood. 



Poisonous substances are formed in some species of Equisetum, and hay with 

 which the shoots are mixed is injurious to cattle. 



CLASS III 

 Lycopodinae (Club Mosses) 



To the Lycopodinae belong, as their most important and widely 

 distributed genera, Lycopodium, Selaginella, and Isoetes. They are 

 distinguished from the other Pteridophyta, of which they resemble 

 most nearly the Eusporangiate Filices, by their general habit and the 

 mode of their sporangial development. 



Unlike the fertile leaves of the Filicinae and Equisetinae, which 

 always bear numerous sporangia, the sporophylls of the Lycopodinae 

 produce the sporangia singly, at the base of the leaves or in their 

 axils. Although in many cases scarcely distinguishable from the sterile 

 leaves, the sporophylls are frequently distinctively shaped, and, like 

 those of Equisetum, aggregated at the ends of the fertile shoots into 

 terminal spike-like cones or flowers. Compared with the leaves, the 

 sporangia are relatively large. They are developed in the same way as 

 those of the Eusporangiate Filices and Equisetinae, from a projecting 

 group of cells derived from the epidermis and the underlying tissue ; 

 while in the other Pteridophyta the sporangia are developed from a 

 single epidermal cell. The innermost layer of the sporangial wall, 

 the tapetal layer, is not absorbed. The sporangia have no annulus. 

 Except in the case of Isoetes, whose spores become free by the decay 

 of the sporangial wall, they dehisce by longitudinal slits, which divide 

 the sporangia into two or more valves ; the slits occur where rows of 

 cells of the wall have remained thin ( m ). The sporangia of Lycopodium 



