SPORE-PRODUCING MEMBERS 



379 



A. 



Each of the sporangiophores, from which the sporangia thus depend, 

 is traversed from the stalk upwards by a vascular strand, which brunches 

 in the enlarged head, and each branch terminates immediately below the 

 base of one sporangium. The sporangiophores are in close juxtaposition 

 while young, and thus the sporangia are effectively protected. At the base 

 of the strobilus lies the annulus, which completes the investment of the 

 lowermost series of sporangiophores : it has as a rule no vascular supply 

 (Fig. 209). Goebel has pointed out the 

 protective biological use of the annulus ; l 

 also that at the apex the highest spor- 

 angiophores may be imperfectly developed 

 and concrescent, thus forming a terminal 

 cap : the protection of the young sporangia 

 is thus very complete. 



The number of sporangiophores in the 

 Egmsetum-stioibilus is not strictly defined, 

 while the number of sporangia on each 

 sporangiophore is also variable : it is usually 

 larger in Equisetum than in the Cala- 

 marians: this raises the question of evidence 

 of variability of number of sporangia. There 

 is in Equisetum no structural evidence of 

 the septation of sporangia such as might 

 lead to their increase in number, nor is 

 'there any interpolation of later sporangia 

 between those first formed. In some of 

 the larger cones, such as E. maximum, 

 branched sporangiophores are commonly 

 found, which appear to indicate a possible 

 increase in their number by fission : the 

 irregularity of their number and arrange- 

 ment in these large cones would seem to 

 support this (compare Fig. 195.) Excepting 

 for such indications there is no evidence 

 among living species of methods of increase 

 in number of sporangia. Even the apical growth of the strobilus itself is, 

 as a rule, strictly limited. Of reduction in number of sporangia there is 

 as little direct evidence, but it is to be remembered that complete abortion 

 leaves no trace of what has occurred (see Chapter X.). On grounds to be 

 mentioned below it would seem probable that such complete abortion of 

 sporangiophores has figured in the evolution of the Equisetales, contributing 

 to the origin of the initial vegetative system of the individual plant. 



The structure of the mature sporangiophore and of the sporangia in 

 the Calamarians is so similar to that of Equisetum that, taken together 



1 Organ'ography, ii., p. 500. 



ameiisf, L. A, .section tra- 

 versing a sporangium in median longitudinal 

 plane: the cells marked (x), like those in 

 Fig. 206, are traced in origin (Yum segmen- 

 tation of superficial cells subsequent to the 

 first periclinal division. B, a similar spor- 

 angium cut transversely. X 200. 



