424 



TRANSFORMATION OF ENERGY 



Fig.128. Pilobolus, Dia- 

 grammatic longitudinal 

 section. /, upper end of 

 the sporangiophore ; y, 

 line of rupture ; sp, 

 sporangium. After De 

 Bary (1884). 



There is another series of examples which may serve as an illustration of 

 ejaculatory movements, which are due not to the activity of a single cell but 

 to tensions in many. The method met with in the squirting cucumber {EchalUum 

 elatenum) (Hildebrand, 1873) reminds one of the phenomena of ejection as 

 illustrated hy A scoholus. The elongated ovoid fruit (Fig. 129), owing to the 

 bending of the peduncle, turns its base upward. It consists of a wall formed of 

 several layers of cells, enclosing slimy contents enveloping the seeds. When 

 the fruit is ripe, as is indicated by its turning yellow, the part of the peduncle 

 nearest to the fruit wall becomes loose, and if the fruit be lightly touched it springs 

 out of the fruit-wall just like the cork out of a champagne 

 bottle (Fig. 129, //). At the same moment the mucilaginous 

 contents of the fruit, together with the seeds, are ejected 

 with great force to a considerable distance. It is obvious 

 that the fruit-wall contracts during this process and the 

 amount of contraction may readily be measured. Thus 

 a contraction in length from 100 to 86, and in diameter 

 from 100 to 84, has been observed in a fruit not perfectly 

 ripe, and in all probability a still greater contraction occurs 

 in the absolutely ripe condition, but owing to the readiness 

 with which such fruits explode it is by no means easy to 

 carry out measurements on them. The fruit-wall must, 

 therefore, have been in a stretched condition previous to 

 the bursting. Researches hitherto made on Echallimn 

 have not determined whether this stretching, as in the case of A scoholus, is due 

 to osmotic pressure or to pressure resulting from the swelling of the contents. 

 On the contrary it appears to us improbable that the pressure of the fruit wall 

 itself operates in the way Hildebrand suggests. According to this author 

 the external layers of the wall, which are composed of large, succulent, thin- 

 walled cells, are able to extend more than the inner layers, and must in conse- 

 quence exert pressure on the interior of the fruit and so bring about the 

 explosion. As a matter of fact, however, the layers of the fruit-wall including 

 the outer ones, as Dutrochet (1837) long ago observed, shorten during the con- 

 traction of the fruit-wall. 



In the majority of cases, certainly, stretching is induced in the fruit-waU in 

 the way^ in which Hildebrand believed it takes place in 

 EchalUum, that is, by differential stretching of different cell 

 layers, by tissue expansion. As an example of this we may take 

 the case of Inipatiens (Eichholz, 1885). The fruit is composed 

 of five carpels, and the seeds arise from an axile placenta. 

 When the fruit is ripe the five thin lateral walls separate, not 

 only from the placenta but also from the outer wall, and this 

 latter splits into five valves, each corresponding to the region 

 between two lateral walls. When fully ripe the least touch 

 is sufficient to isolate the five valves from each other, and 

 each of these rolls itself up suddenly, like a watch-spring, 

 beginning at the base, striking against the seeds, and throwing 

 them out. If one attempts to bend the valve straight again, 

 it breaks across, but if it be plasmolysed all opposition to 

 the straightening is removed. Hence it may be concluded that 

 we are dealing here with an osmotic phenomenon, and closer 

 investigation demonstrates that a layer of parenchyma lying 

 under the external epidermis acts dynamically, swelling, and 

 to it the internal cell mass acts antagonistically. In the 

 complete fruit, this layer is positively stretched, it endeavours to expand, and 

 this at once results in the isolation of the valves, because its cells possess very 



Fig. 129. Fruit 

 of EchalUum Ela- 

 terium. /, in lon- 

 gitudinal section. 

 U, open at the 

 top. After Hilde- 

 brand (1873). 



