338 SPOROPHYLLS OF EQUISETUM 



are not favorable for the work of the green shoots. Note in this 

 connection the significance of the coloration. Species with the 

 strobili upon the ordinary plants form these structures later in 

 the season when the temperatures are favorable for photosyn- 

 thesis. The development of the sporangia upon specialized 

 sporophylls that are grouped together at definite points on the 

 plant is one of the significant departures that appears in the evo- 

 lution of the plant. This distribution of labor that is seen in the 

 setting aside of certain groups of leaves for spore production 

 will appear.in all the succeeding groups ; and the arrangement and 

 structure of the sporophylls will steadily become more and more 

 complex until a point is reached where they are popularly called 

 a flower, although' this term can be just as correctly applied to 

 the strobilus appearing in Equisetum and succeeding groups. 

 The sporophylls of Equisetum originate at the nodes, as in the 

 case of the scale leaves, but as they enlarge the apical portion 

 spreads out like a shield. The internodes do not elongate and 

 separate the sporophylls to any considerable extent, consequently 

 the shields become six-sided through contact with the adjacent 

 sporophylls. 



(b) Character of Sporangia and Spores. — The sporangia appear 

 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 

 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 



