FERNS AND HORSETAILS 199 



two rows of large spore-cases or sporangia sunk in the tissue. At 

 maturity the sporangia open by transverse slits and discharge the 

 inclosed spores. 



When the spores germinate they produce subterranean tuberous 

 prothallia which, however, are rarely found, and of whose history little 

 is known. They develop archegonia and antheridia beneath the surface 

 of the ground, and the fertilized egg produces the young fern plant. 



The generations of the true ferns are explained in Chapter XXVI. 



EQUISETUMS, OR HORSETAILS 



There are about twenty-five species of equisetum, constituting 

 the only genus of the unique family Equisetacese. Among these E. 

 arvense is common on clayey and sandy soils. 



In this species the work of nutrition and that of spore-production 

 are performed by separate shoots from an underground rhizome. The 

 fertile branches appear early in spring. The stem, which is 3 to 6 

 inches high, consists of a number of cylindrical furrowed internodes 

 each sheathed at the base by a circle of scale-leaves. The shoots are 

 of a pale yellow color. They contain no chlorophyll, and are nourished 

 by the food stored in the rhizome. (Fig. 369.) 



The spores are formed on specially developed fertile leaves or 

 sporophylls which are collected into a spike or cone at the end of the 

 stalk (Fig. 369, a). A single sporophyll is shown at 6. It consists 

 of a short stalk expanded into a broad, mushroom-like head. Several 

 large sporangia are borne on its under side. 



The spores formed in the sporangia are very interesting and beau- 

 tiful objects when examined under the microscope (X about 200). 

 They are spherical, green bodies each surrounded by two spiral bands 

 attached to the spore at their intersection, s. These bands exhibit 

 hygroscopic movements by means of which the spores become entangled, 

 and are held together. This is of advantage to the plant, as we shall see. 



All the spores are alike, but some of the prothallia are better nour- 

 ished and grow to a greater size than the others. The large prothallia 

 produce only archegonia while the smaller ones produce antheridia. 

 Both of these organs are much like those of the ferns, and fertilization 

 is accomplished in the same way. Since the prothallia are usually 

 dioecious, the special advantage of the spiral bands holding the spores 

 together, so that both kinds of prothallia may be in close proximity, 

 will be easily understood. As in the fern, the fertilized egg-cell develops 

 into an equisetum plant. 



