SPORE-PRODUCING MEMBERS 



377 



in the middle of the internode^(Fig. 202), and in Stachannularia or 

 Cingularia at the top of the internode (Fig. 204). Such facts as these, 

 here only briefly sketched, must be taken into account in discussing the 

 morphology of the strobilus of the Equisetales, and in deciding the true 

 chararacter of the sporangiophores. But before this is entered upon their 

 detailed structure and development must be examined. 



SPORE-PRODUCING MEMBERS. 



Naturally the development of the spore-producing members can only 

 be followed in the living genus, though from the similarity of their mature 

 features to those seen in the fossils it 

 is probable that there was substantial 

 similarity in these also. In Equisetum 

 the axis, which is about to produce 

 a strobilus, ceases active growth in 

 length, retaining a conical form : the 

 sporangiophores arise upon it in aero- 

 petal order, as convex swellings (Fig. 

 205). The details show some varia- 

 tion in different species : they are here 

 described for Equisetum arvense and 

 Itmosum* In the first stages the spor- 

 angiophores are not unlike the sterile 

 leaf-sheaths, involving, as seen in 

 longitudinal section, some six cells, 

 which grow out with a fan-like tracery 

 and repeated anticlinal walls (Fig. 

 206 A). This similarity has been used 

 as an argument favouring the view that 

 the sporangiophore and the bract-leaf 

 are results of " metamorphosis " of 

 essentially the same part, a point 



which will be taken up later. Single superficial cells near the margins 

 of the convex outgrowths' are early recognisable as the parent cells which 

 give rise to all the essential parts of the sporangia, though adjoining 

 cells also grow out together with these* to form the sporangial body : the 

 origin of the sporangium is thus of the eusporangiate type (Fig. 206 A, B). 

 At an early stage there is active growth in the middle region of the 

 sporangiophore, which results in an inversion of the young sporangia, so 

 that they come to point with their apices towards the axis. Each parent 

 cell first divides periclinally (Fig. 206 A) : the inner cell gives rise only 

 to a portion of the sporogenous tissue, the outer undergoes further division, 

 first by anticlinal, later by periclinal walls (Fig. 206 B, c, D). The inner 



1 Studies, i. , p. 496, etc. 



FIG. 205. 



Half-developed strobilus of Equisetum arvense, 

 in longitudinal section, taken at end of October. 

 X 50. (After Hofmeister.) 



