SPECIAL AND GENERAL 149 



contents. When the latter have escaped the wall of the fruit contracts 

 considerably, and the escape is prepared for by the loosening of the 

 tissue round the top of the fruit-stalk. A similar spurting mechanism is 

 responsible for the escape of the spores from the spore-sacs of many 

 Discomycetes, Pyrenomycetes and Lichens, for the spores, together with 

 a portion of the unused contents of the sporangium, may be thrown out 

 sometimes to a distance of several centimetres 1 . After dehiscence the 

 wall of the ascus distended by turgor may contract to three-quarters or 

 two-thirds of its previous length, as when a short rubber-tube distended 

 with water is pricked and the contents allowed to escape. A similar con- 

 traction is naturally also shown by the ascus when its contents are 

 plasmolysed. 



In certain Pyrenomycetes, previously to the dehiscence of the ascus, 

 its outer cuticular wall ruptures and the distensible inner wall elongates to 

 as much as twice its original length, so that the apex of the ascus reaches 

 to or protrudes beyond the narrow mouth of the fructification 2 . (Fig. 34.) 

 In some cases the spores all collect at the apex and are thrown out 

 simultaneously, but in other cases they follow one another. Each blocks 

 the apex for a while until the turgor has risen sufficiently to throw it out, 

 when another blocks the narrow opening and, after a pause, is thrown out 

 in its turn, when the turgor is once more restored. The fact that the 

 dehiscence takes place at a definite point shows either that the membrane 

 has remained weaker here than elsewhere or else that the protoplasm 

 has produced a diminution of the cohesion of the wall at this point pre- 

 viously to dehiscence. In the former case a rise of turgor would be 

 required to produce dehiscence, but not necessarily in the latter. It is, 

 however, not surprising that shaking or changes in the moistness of the 

 air may excite or accelerate the dehiscence. 



In many cases a pronounced swelling of the wall takes place, which 

 may aid in producing dehiscence and in narrowing the cavity of the ascus. 

 Prior to dehiscence the swelling is possibly prevented by the pressure 

 exerted by the contents on the wall. The rupture of cuticular membranes 

 is by no means uncommon and occurs normally whenever the inner walls 

 continue to grow, or when cuticularized gland-cells are actively excreting. 

 The threads which escape from the grandular hairs of Dipsacus under water 

 are probably extended through cracks in the cuticle. They appear to 

 be products of the metamorphosis of the cell- wall, and their peculiar move- 

 ments are probably similar in character to those shown during the formation 

 of myelin threads 3 . 



1 De Bary, Morphologic u. Biologic d. Pilze, 1884, p. 90 (Fungi, Mycetozoa and Bacteria) ; 

 Zopf, Die Pilze, 1890, p. 87 ; Ludwig, Biologic der Pflanzen, 1895, p. 328. 



2 Pringsheim, Jahrb. f. wiss. Bot., 1858, Bd. I, p. 190. 



3 Cohn, Bot. Ztg., 1878, p. 123; F. Darwin, Journal of Microscopical Science, 1877, Vol. xvil, 

 p. 245, and 1878, Vol. xvni, p. 73. 



