DEVELOPMENT OF PLANTS 



307 



pear as rather elongated sacs on the under side of the shields 

 (Fig. 234, 2) and at maturity open by a longitudinal cleft. The 

 structure of the spores is of especial interest. The outer wall 

 of these spores is thickened in spiral bands, and owing to the 

 dissolution of the thin portion of the wall separating these bands, 

 the outer coat uncoils and appears as four bands, with spoon-like 

 ends, attached to the spore at one point (Fig. 234, 3). These 



Fig. 234. 



Fig. 235. 



Fig. 234. Sporophylls and spores of Equisctum: 2, sporophylls viewed 

 from outer and inner side, showing form and attachment of sporangia and 

 the central stalk attaching the sporophyll to the strobilus. 3, spores with 

 elaters expanded in a and partially coiled in b. 



Fig. 235. Female gametophyte of Equisctum bearing several archegonia 

 and leaf-like lobes. At right male gamete. — After Sadebeck. 



bands or elaters are very hygroscopic and their movements assist 

 in rupturing the sporangium, but their special significance is seen 

 in the fact that the elaters become entangled and so several spores 

 are carried away together by the wind. The meaning of this 

 arrangement will appear directly. 



(c) Germination of the Spore. — The gametophyte produced 

 from these spores (Fig. 235) is an irregularly lobed thallus more 

 suggestive of the irregular thallus of an hepatic or the leaves 

 of a moss plant, or the lobed gametophyte of certain species of 

 the Ophioglossales than of the prothallium of the Filicales. The 

 most important feature about the gametophytes is the fact that 

 they are as a rule dioecious and of two sizes, the smaller ones 

 bearing only antheridia and the larger only archegonia. This 

 difference in the nature of the gametophytes is largely due to 



