THE SPHAEROBOLUS GUN 321 



being a lipochrome and one of the carotins. 1 The exposure of the 

 pigment to sunlight just as the fruit-body is most actively preparing 

 to discharge its projectile suggests that the pigment is not a mere 

 waste product of metabolic activity but plays some part in the 

 development of the discharge mechanism. Light, as observed by 

 Miss Walker, 2 not only hastens the opening and discharge of the 

 fruit-bodies but increases the vigour with which discharge takes 

 place. As already suggested, light may hasten the transformation 

 of the glycogen in the palisade cells into sugar. If this is so, it 

 may well be that the transformation process is in some way 

 facilitated by the absorption of light by the orange-yellow 

 pigment. 



The supposition that the osmotic pressure in the cells of the 

 palisade layer is, in the main, the force which causes the ejection of 

 the glebal mass is supported by what we know concerning the 

 relations of the fruit-body with water. The dependence of the 

 discharge mechanism of Sphaerobolus on water supply will now be 

 discussed. 



In contrast with the explosive fruits of Geranium palustre, Viola 

 canina, and many Mimosaceae, Caesalpiniaceae, Sterculiaceae, 

 Acanthaceae, etc. (vide the Table on p. 337), where the expulsion 

 of the seeds takes place as a result of desiccation and the contraction 

 of a special layer of the fruit-wall, the Sphaerobolus gun can dis- 

 charge its projectile only when it is sufficiently supplied with 

 water. There can be no doubt that, at the moment of discharge, 

 the palisade layer of the peridium is in a highly turgid condition. 

 If a mature fruit-body of Sphaerobolus, just before or just after 

 opening stellately, be placed in an atmosphere saturated with 

 water vapour, sooner or later it will eject its glebal mass ; but if, 

 on the other hand, it be allowed to dry, no discharge takes place, 

 the discharge mechanism being rendered inoperative as soon as the 

 osmotic pressure in the individual cells of the palisade layer becomes 

 reduced below a certain minimum. 



1 Cf. J. Zellner, Chemie der Hoheren Pilze, Leipzig, 1907, pp. 12, 139-142. 



2 L. B. Walker, " Development and Mechanism of Discharge in Sphaerobolus 

 iowensis n.sp. and S. stellatus Tode," Journal of the Elisha Mitchell Scientific Society, 

 U.S.A., Vol. XLII, 1927, p. 156. Cf. L. B. Walker and E. N. Andersen, loc. cit., 

 pp. 156-157. 



VOL. V. V 



