364 XXVI. REPRODUCTION OF THE PTEBIDOPHYTA. 



green, or of special colourless shoots. The sporangiophores are 

 metamorphosed leaves, arranged in whorls, and, by mutual lateral 

 pressure, have assumed a polygonal, usually hexagonal, form. 

 In order to study them more closely we first remove a number of 

 them with a scalpel from the axis of a ripe sporangiferous shoot, 

 and examine them dry, with direct illumination, under the simple 

 microscope. All species are practically alike ; we will, however, 

 select the common Equisetum limosum of ditches, fructifying in 

 June and July. 



In each sporangiophore it is easy to distinguish the poly- 

 gonal shield and the stalk which bears it. On the inner surface 

 of the shield, surrounding the stalk, arise about eight sac-like 

 sporangia, w r hich, in order to evacuate their spores, split along 

 the entire length of the side directed towards the stalk. The 

 internal structure of the sporangiophore and the sporangia can be 

 seen upon cross-sections, taken through an axis which is not quite 

 ripe. Decidedly more favourable results will be attained by the 

 use of alcohol material, softened and examined in glycerine. The 

 stalk of the sporangiophore is traversed by a central vascular 

 bundle. At its apex it broadens out into the shield, and its 

 vascular bundle divides, umbrella-wise, into just so many strands 

 as there are sporangia present. The bundle-branchlets end with 

 spirally-thickened tracheides under the place of insertion of the 

 sporangia. The epidermis of the sporangia is distinguished by 

 the beautiful spiral, or in part annular, thickening of its cells. 

 The sporangial wall appears reduced to this epidermis, and to 

 some ultimately-collapsed layers of cells. During their formation 

 the spores lie embedded in an epiplasm, very rich in starch, and 

 fixed by alcohol, which is consumed during the later development 

 of the spores. We will study the spores of E. limosum in fresh 

 material. These are distinguished at once by their striking 

 elaters. These elaters are two bands, derived from the cleft 

 outer membrane of the spore. They hold together only at one 

 spot, in rectangular fashion, and form therefore a four-armed 

 cross, whose arms are rolled around the spore. At their end, the 

 bands are expanded spathulately. These bands are very hygro- 

 scopic ; upon drying they unroll, in moist air they roll up again. 

 If we breathe upon dry spores lying (uncovered) upon an object- 

 slide, and while observing them under the microscope, the bands 

 will be seen to begin to roll up, whereby the whole mass of spores 

 is set in movement. The use of this mechanism lies in the hook- 



