HORSE TAILS 161 



antheridia (singular antheridium). In some ferns they are 

 borne on the same prothalhis while on others they are borne 

 on different prothallia. 



The Archegonia (Fig. 103, V) are borne near the notch of 

 the prothallus, and somewhat flask-shaped with the base imbed- 

 ded in the tissues and the neck extending downward and slightly 

 curved. An examination with the compound microscope shows a 

 large egg (ovum) or female cell (Fig. 103, c") in the large part 

 and a row of canal cells in the neck. These canal cells become 

 gelatinous or semi-fluid in character. 



The Antheridia (Fig. 103, 6') are spherical and produce 

 great numbers of minute, unicellular, spiral, free swimming 

 bodies known as sperms or male cells. (Fig. 103, c'.) The 

 sperm cells swim in the moisture on the surface of the prothallia 

 and the soil, and some few eventually reach the archegonia, enter 

 the canal and one unites with each ovum or egg cell. This is 

 known as fertilization. It corresponds to the fertilization of 

 the egg in the embryo sac of the Angiosperm. (Chapter VI.) 

 As a result of this fertilization, cell division occurs and the 

 egg grows into a fully developed fern, (Fig. 103, e, /.) With the 

 development of the fern, the prothallus, upon which it at first 

 feeds, is gradually destroyed. 



Horse Tails. There are many other fern-like plants which 

 we will find quite interesting if we have time to study them. 

 Among the most common are the horse-tails or scouring rushes 

 (Equiseturri). The aerial stems are derived from an under- 

 ground stem and may be branched or unbranched, hollow jointed, 

 fluted with well-marked longitudinal ridges and furrows and 

 infiltrated with silica which makes them rough and rigid. The 

 branches and modified leaves are always in whorls and at the 

 nodes. At each node the stem is surrounded with membran 

 ous sheaths which correspond to the leaves of higher plants. A 

 cone-shaped fruiting structure is borne at the apex of the stem 

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