THE PTERIDOPHYTA : LYCOPSlDA, ETC. 



629 



the outer surface only, and they twist and turn according to the amount of 

 moisture in the air. In moist air they curl up, but when they are dry they 

 stretch out again. There is some doubt as to whether these structures assist 

 in the dispersal of the spores, but it seems certain that they materially aid in 

 effecting the dehiscence of the sporangium. The spores contain a number of 

 chloroplasts, and as is usual in similar cases only remain viable for a few days. 

 The spores of E. arvense are apparently all alike, as in Lycopodhim, and 

 the species is said therefore to be homosporous. Careful observation, 

 however, shows that there are two kinds present. If a large number are 



25 26 27 



42 43 44 45 46 47 48 49 50 u 



18 29 30 31 32 33 34 35 36 37 33 39 40 41 



Diameters in Micra 



Fig. 638. — Equisetum arreme. Variation of spore size. The smaller and larger spores 

 have an almost independent range in variation of diameter, the curves havmg 

 widely separated modes. 



measured and their diameters plotted on squared paper it will be found that 

 two curves are produced, with well-separated maxima and very little overlap 

 (Fig. 638). The smaller series are much paler green than the larger ones 

 and can be picked out by eye in a fresh sporangium. The two kinds are 

 not equal in numbers nor is the number of each type constant in a tetrad. 

 This is not a perfect heterospory, because though the small spores produce 

 small male prothalli, the large spores produce hermaphrodite prothalli, i.e., 

 with both male and female organs. The case is interesting, however, as 

 showing apparently an early stage in the development of heterospor}'. Some 

 other species of Equisetum, especially those with sporangia on the green 

 vegetative stems, show no difference of size among their spores. Some 

 of the fossil Calamites, on the other hand, which were homosporous (C. 

 binueyana), do show a corresponding difference of size, while other Calamite 

 species had definitely developed true heterospor\' (C casheauo). 



