268 



THIRD GROUP.— VASCULAR CRYPTOGAMS 



brought about, as by breathing lightly on the spores under the microscope, the latter 

 are set in lively motion by the movements of the elaters. The office of the elaters 

 is to cause a number of spores to remain attached to one another at the period of 

 dispersion, so that they leave the sporangium in small groups ^ ; if they become wetted, 

 as they may on slightly moistened ground, the union between them becomes still 

 closer, since the elaters which are only partially rolled up hold them all the tighter 

 together. As the prothallia are generally unisexual, it is evidently advantageous 

 to the plant that the elaters should cause several spores to germinate in the 

 immediate neighbourhood of one another. If spores, in which the outer coat is 

 not yet split up to form the elaters but shows the preliminary differentiation, are 

 allowed to lie for some time in glycerine, each spore surrounded by its third (inner) 

 coat shrinks considerably, and the second cuticularised coat separates from the inner 

 in folds. The third (inner) coat is differentiated into an outer granular cuticularised 

 exosporium, and an inner layer of cellulose, the endosporm?n. 



Fig. 223. A longitudinal section through a portion of a sporophyll (sporangiophore) oi Equisetum palustre; 

 sporangium is being formed, the archesporium shaded. B an older sporangium in axile longitudinal section, 

 /, t tapetal cells. The mass of sporogenous cells formed from the archesporium consists at first of a few cells only. 



There is little to be said in this place on the subject of the classification of the Equise- 

 taceae ; all existing forms in this family are near enough to each other to be included in 

 one genus, Eqidsetiim, and to this genus the numerous fossil species may also be 

 united. 



In habit as in morphological character the Equisetaceae are very distinct from almost 

 all other plants. They are perennial by means of underground creeping rhizomes ; 

 these send up shoots every year which rise erect above the surface of the ground and 

 there last usually during one period of vegetation, more rarely for several years. The 

 sporangiferous spikes appear at the summit of these vegetative assimilating axes or on 

 special fertile shoots, which when without chlorophyll and unbranched die down after 



' This was demonstrated by De Bary in Bot. Ztg. 1884, p. 782. The name ' elater ' is therefore 

 quite unsuitable. With regard to the origin of the membrane which produces the elaters, opposing 

 views have been published ; that taken in the text is founded on the researches of Sachs ; see also 

 Russow, Vergl. Unters. p. 149. 



