PTERIDOPHYTES 313 



These xerophytic characteristics are apparent in the minute, 

 scalelike leaves, the highly protected epidermis of the stems, 

 and the poorly developed vascular system in the horsetails, 

 all of which are in strong contrast to the large leaves and 

 highly organized vascular system of their mesophytic allies 

 the true ferns. The plant body in the equiseta is composed of 

 an underground perennial stem, or rhizome, which serves the 

 function of storing food and of hibernation when the aerial 

 parts of the plant are destroyed by cold or drought. This 

 perennial rhizome gives rise to annual green shoots, which are 

 simple in some species and highly branched in others. These 

 aerial stems, since they produce distinct chlorophyll-bearing 

 tissue, serve the double function of making starch by photo- 

 synthesis and of bearing asexual spores. The plant body thus 

 manifests the same physiological division of labor as we have 

 already noted in the ferns, but the work of photosynthesis and 

 spore bearing is performed in the equiseta by the annual shoots, 

 instead of by the leaves as in the ferns. 



Asexual reproduction. The spores are borne in strobili, or 

 cones, which terminate either the regular green shoots or spe- 

 cial reproductive branches as in Equisetum arvense (Fig. 179). 

 The reproductive cones (c) consist of a number of shield-shaped 

 sporangiophores ((7), each of which bears several sporangia, shaped 

 like the finger of a glove. When the spores are ripe, the axis 

 of the cone, or strobilus, lengthens, thus separating the spo- 

 rangia, which then open along a seamlike layer of cells on one 

 side and liberate the spores. Each spore (.Z?) is furnished with 

 four appendages, called elaters, which open out in dry weather 

 and close up around the spore in a moist atmosphere. When 

 spread out, the elaters assist in the distribution of the spores by 

 air currents. The spores germinate much as in the ferns and 

 produce a gametopJiyte plant, which bears the gametes. The 

 peculiarity of the spores of Equisetum in this regard is that some 

 spores produce gametophytes which bear antheridia only, white 

 others produce only female gametophytes. The spores are alike 

 in form and are therefore said to be homosporous ; but they are 

 evidently differentiated physiologically, since they produce either a 



