en YP TOGA MO [IS TYPES. 



them ; or, in other cases, the protliallia maybe dioocious. 

 Fertilization of the Kerni-coll, whir-h occupies a cavitj' at 

 the base of the arcliegonium, takes jjlace exactly as in thu 

 Ferns, and, as a result of fertilization, the germ-cell 

 developes into a spore-bearing plant similar to the origi- 

 nal one. Here, therefore, we have again exhibited an 

 alternation of generations. 



Other species of Equisetiuu of common occurrence, instead of 

 producing a special fertile branch, dcvelope sporangia at 

 the extremities of the ordinary leafy stems. 



These plants, like the Ferns, exhibit fibro-vascular bundles, 

 and the epidermis is specially characterized by the exces- 

 sive amount of silica contained in it, some of the specifs 

 being used for scouring and polishing by reason of this 

 property. 



The curious elaters{F\Q. 301) attached to the spores doubtless 

 assist them to escape from the spore-cases, and subse- 

 quently aid in dispersing them. 



Club-Mosses. 



Fig. 302 is a representation of a branch of LijcopoiUum 

 clavatum, one of our common Club-Mosses. The creep- 

 ing stem lies flat upon the ground, and often attains a 

 great length, sending up at intervals erect branches with 

 crowded linear-awl-shaped leaves, some of which, like 

 the one shown in the figure, are terminated by a slender 

 peduncle bearing one or more cylindrical spikes. Tiiese 

 are the fertile branches, and the leaves upon them, or at 

 all events upon the slender upper part, are very much 

 smaller than upon the ordinary' sterile branches. 



It is to be observed that the stems and roots of these plants 

 branch duhotomously. 



The sporangia are produced in the axils of the leaves of the 



terminal spike. One of these leaves greatly magnified, 



with its attached sporangium, is shown in Fig. 303. 



The sporangium opens by a slit at the top to discharge 



the spores. 



It is onlj^ quite recently that the prothallium has been detected. 

 It is described in the case observed as a "yellowish-white 

 irregular lobed body, sparingly* furnished on its imder sur- 

 face with small root-hairs." The antheridia and arche- 

 gonia apjiear to be produced on the upjicr surface, and 

 these b}' their interaction, give ri.se to the new plant which 

 bears the spores, just as in the Ferns and Hor.setails ; so 

 that again there is an alternation of generations. 



It is a fact of great interest that in some plants nearly related 

 to the Club-Mosses, two kinds of spores— large and small 

 — are jiroduced in separate sporangia. The large ones 



Fig. 801. 



Fig. 308. 



Fijr. aar 



