PRINCIPAL GROUPS OF PLANTS. 97 



polishing woods, and Equisetum arvense is used for scouring 

 tinware. 



LYCOPODIALES. 



The Lycopodiales, or Club Mosses (Fig. 66), are perennial 

 moss-like plants, with more or less erect or creeping and branching 

 stems, on which are borne numerous small simple leaves. The 

 sporangia arise either at the base of the upper surface of the leaves 

 or occur in terminal cones. They have short stalks, are uni- 

 locular and 2-valved. The asexual spores are of one kind in 

 Lycopodium and in the form of spherical tetrahedrons, resulting 

 from the manner in which division has taken place (see Vol. II). 

 In Selaginella (Fig. 60) two kinds of asexual spores are produced, 

 that is, both microspores and megaspores, which in turn give rise 

 to male and female prothalli respectively. The microspore develops 

 a male gametophyte (Fig. 62) which remains entirely within the 

 spore, and consists of a few-celled prothallus and a number of 

 mother cells which produce sperms that eventually escape by the 

 breaking of the wall. 



The megaspore frequently begins to develop the gametophyte 

 (Fig. 63) while still within the sporangium. The prothallus con- 

 sists of a number of cells and partly protrudes through the rup- 

 tured spore wall. On the upper part of the prothallus or nutri- 

 tive layer a few archegonia are borne. It should be stated that 

 sometimes the archegonia are developed very early on the pro- 

 thallus tissue, but usually they are developed after the spores 

 have escaped from the sporangium. After fertilization of the egg 

 a multicellular embryo develops which shows the following parts 

 (Fig. 61) : (i) An elongated cell or row of cells which extends 

 into the tissues of the prothallus for the purpose of obtaining 

 nutriment; (2) a root; and (3) a stem bearing at its tip two 

 leaves, or cotyledons. One of the specially notable characters 

 of the plants of the Selaginella group is, as we have seen, the 

 great reduction in size of the gametophyte, which in the case of 

 the microspore does not enlarge beyond the wall of the spore, and 

 in the case of the megaspore only partly protrudes beyond its wall. 



Isoetes. This is a genus of aquatic or marsh plants known 

 7 



