THE SPHAEROBOLUS GUN 317 



" Thus, by elimination, the force necessary for the discharge 

 must be confined to the inner peridial region. We can easily see 

 that in the structure of the palisade layer with its very unusually 

 long, narrow, radially placed cells, free on their inner ends and closely 

 bound together on their outer ends by the layer of small tangential 

 hyphae (c/. Fig. 162, A), we have an ideally constructed mechanism 

 for producing the discharge. The palisade cells, if turgid, would 

 tend to expand on the inner end while held closely together on the 

 outer end by the tangential filaments. That these cells are extremely, 

 almost explosively, turgid is evident when free-hand sections of 

 fresh materials are cut at the time of the discharge or just after the 

 discharge. Faint pops may even be heard as the cells are cut." 



As we have seen from the remarks just quoted, Miss Walker con- 

 siders that both of the inner membranes play a part in bringing 

 about eversion and the discharge of the glebal mass : the inner 

 fibrous layer is a non-expanding basal plate to which the inner 

 ends of the elongated palisade cell? are attached, while the palisade 

 layer is an expansive layer which is free to increase its area only on 

 its upper surface. 



It seems to me that the fibrous layer, on account of its thinness 

 and the looseness of the individual hyphae of which it is composed, 

 can play but a relatively minor role in eversion ; and it may well be 

 that, if the fibrous layer were to be removed from the palisade layer, 

 the isolated palisade layer might evert on its own account. The 

 palisade layer is relatively very thick, and its cells are firmly 

 attached to one another so that there are no intercellular spaces 

 between them (Fig. 162). My study of the palisade layer inclines 

 me to believe that the layer is so organised that its upper cells or 

 the upper parts of its much -elongated cells which are more or less 

 pear-shaped tend to expand more than its lower cells or the lower 

 parts of its elongated cells and that, in consequence, the palisade 

 layer tends to expand far more on its upper surface than on its lower 

 (cf. A and B in Fig. 162). Thus it seems to me that, just before the 

 discharge of the Sphaerobolus gun, in the U-shaped pocket, the 

 upper part of the palisade layer is in a state of great compression 

 while the lower part of the palisade layer is in a state of great tension. 

 In the opened fruit -body, as expansion takes place by the rim 



