REPRODUCTION OF EQUISETUM 



313 



and bears on the inner face of its hexagonal lamina a ring of 

 5 to 10 sporangia encircling the stalk (Fig. 176, B). In the young 

 cone the heads of the sporophylls fit closely together, thus 

 forming a compact protection for the sporangia; but as the 

 latter mature the sporophylls separate, through elongation of 

 the axis, and permit escape of the green spores. The individual 

 sporangia are rather larger than those of Ferns, and have a 

 several-layered wall. Many of the fossil Horsetails possessed 

 cones of more elaborate 

 structure. 



At maturity the 

 outermost coat of each 

 spore is seen to consist 

 of four extremely hygro- 

 scopic spiral strips (Fig. 

 176, C, D) which only 

 remain attached at one 

 point. Groups of spores 

 consequently tend to 

 cling together when the 

 contents of a sporangium 

 are scattered by the 

 wind, and this may be 

 of importance, since the 

 archegonia and anthe- 

 ridia are usually pro- 

 duced on distinct 

 prothalli. The spores 

 are, however, all alike, 

 the sex of the resulting 

 prothalli depending on the conditions of nutrition, those poorly 

 nourished becoming male, whilst those well nourished become 

 female. Both kinds are more or less richly branched, but the 

 male are much smaller than the female. The sexual organs 

 borne upon them do not differ essentially in construction from 

 those of Ferns. It may be noted that, when Fern prothalli 

 grow densely crowded, they often bear antheridia only, although 

 unisexuality is here the exception rather than the rule. 



The present-day Clubmosses are represented mainly by the 



FIG. 176. A, Entire cone, and B, single 

 sporophyll of Equisetum maximum (ori- 

 ginal). C and D, Mature spores, showing 

 the splitting of the outer coat (C after 

 Sachs; D after Dodel-Port). 



