254 



PLANT MORPHOLOGY 



The first shoots to appear above ground in the spring are fertile but are 

 unbranched, yellowish brown, and lacking in chlorophyll. They wither 

 soon after the spores are shed. Green, branching, sterile shoots then 

 appear and persist throughout the summer. 



The strobilus of Equisetum consists of a central axis bearing numerous 

 whorled sporangiophores (Fig. 211A). The development of sporophylls 



is entirely suppressed. Each sporan- 

 giophore is peltate and bears 5 to 10 

 pendent, sac-like sporangia attached 

 to the margin of a six-sided disk. This 

 is supported by a short stalk that arises 

 directly from the axis of the strobilus 

 and is perpendicular to it (Fig. 2115, 



C). 



The sporangium, which is eusporan- 

 giate in development, arises from a 

 single superficial initial and not, as in 

 the Lycopodiinae, from a row of initials 

 (Fig. 212). However, the sporogenous 

 tissue is derived not only from the 

 inner segment that results from the first 

 periclinal division of the initial but also, 

 in part, from the outer segment (Fig. 

 212D). The tapetum is derived from 

 the wall, which becomes several layers 

 thick but, when the sporangium is ma- 

 ture, consists of a single layer of cells, 

 the inner layers breaking down. In 

 contrast to that of the lycopods, the 

 tapetum becomes two- or three-layered and soon disorganizes, forming a 

 Plasmodium around the spore mother cells. Following the formation of 

 tetrads, the tapetal Plasmodium is absorbed by the developing spores. 

 Equisetum is homosporous. The spores are unique in containing numer- 

 ous chloroplasts when ripe and in having, on the outside, two slender 

 bands derived from the outer layer of the spore wall (Fig. 211D). These 

 are hygroscopic and assist in spore dispersal. Each band is attached to 

 the spore at its middle, the tips being spatulate. The bands uncoil when 

 dry and wrap around the spore when moist. When the spores are ripe, 

 the axis of the cone elongates slightly, separating the sporangiophores. 

 The sporangia dehisce by means of a longitudinal slit that appears on 

 their inner side. 



Gametophyte. The spores of Equisetum are short-Uved and germinate 

 at once. The gametophyte is usually less than 10 mm. in diameter. It 



wr't 



A C 



Fiu. 211. Spore-beariiig structures 

 of Equisetum arvense. A, terminal 

 portion of fertile shoot with strobilus, 

 natural size. B, a single sporangio- 

 phore, X15; C, longitudinal section 

 of same, X15; D, two ripe spores, 

 X150. 



