MOVEMENTS DUE TO TURGOR AND GROWTH 



423 



Ascobolus 

 /, young 



Fig. 126. 

 furfiiraceus. 

 ascus. //, ripe ascus. ///, 

 empty ascus. x 150. 

 After De Bary (Morph. 

 d Pilze. Leipzig, 1884). 



ment a circular patch at the apex of the ascus can no longer withstand the 

 pressure inside, and it forthwith bursts. In consequence of the pressure 

 of the elastic and contractile membrane the entire contents of the cells are 

 at once ejected through the opening, in the case of Ascobolus to a height of 

 seven or more cm., and in Sordaria fimiseda to as much as 15 cm., whHe. the 

 wall at once contracts to its original dimensions. In Ascobolus a large number 

 of asci burst at the same moment in the day, i. e. between one and three o'clock, 

 the extension and extrusion from the hymenial surface having begun the 

 evening before. The ejection occurs then after a very gentle 

 shaking, which probably acts so as to cause a slight bending 

 of the ascus and in this way an increased stretching of the 

 membrane beyond the capacity of the apical region to with- 

 stand ; at the same time it can scarcely be doubted that in 

 the long run ejection may occur without such oscillation. 

 This daily periodicity in the ejection of spores is obviously 

 dependent on light, but into these relationships we cannot 

 enter here. Nor can we discuss in detail the various 

 means whereby asci open, for the isolation of an apical lid 

 as in Ascobolus is by no means general. 



A certain amount of interest, however, attaches to the 

 type of ascus which ejects its spores one at a time, as 

 takes place in many Pyrenomycetes, e.g. Sphaeria scirpi 

 (Pkingsheim, 1858). Before the spores begin to be ejected 

 a sudden extension of the ascus takes place, amounting to as 

 much as three times its original length. As a consequence, 

 the outer lamella of the wall of the ascus is ruptured, and rolls up, while the 

 inner lamella becomes stretched (Fig. 127) ; a tension thus arises between two 

 layers of the cell-wall not previously exhibiting any difference. By the vigorous 

 extension of the ascus the spores (which are in this case multicellular), are 

 pushed towards the apex, and very soon the uppermost spore is ejected through 

 the apical opening. As the ascus contracts it becomes slightly shorter and the 

 next spore at the same time closes the aperture, and so renders any further exit 

 of cell contents impossible. Thereupon ensues a fresh increase in osmotic ten- 

 sion in the ascus, increasing until the second spore is 

 ejected through the narrow aperture, and so on. When 

 all the spores are shed the ascus contracts markedly, and 

 at the same time its wall becomes much swollen, and 

 hence it is very apparent that the ejection is not occa- 

 sioned by osmotic pressure, or at least not by that only, 

 but that swelling of the cell-membrane, may take part 

 in the process. Similarly in many other lower organisms 

 the spores are pressed out of the mother-cell by swelling 

 of certain parts of the cell-wall. 



Osmotic activity is, however, responsible for the 

 ejection of the spores of Empusa and its allies (e.g. 

 Basidiobolus, p. 249) and for the corresponding pheno- 

 menon in the sporangia of Pilobolus. Let us consider the 

 case of Pilobolus crystallinus (compare De Bary, 1884). 

 In this plant we have a sporangiophore {t, in Fig. 128) 

 which is much swollen and whose termination is inserted 



into the base of the sporangium. When the pressure in the sporangiophore 

 has reached a certain height a circular rupture occurs in the membrane at r and 

 the contents of the cell are, owing to the contraction of the wall, ejected just as 

 in the case of the ascus of Ascobolus. The liquid which is ejected from the 

 sporangium in this case may be thrown into the air to a height of a metre. 



Fig 127. sphaeria scirpi. 

 I, extended ascus, the outer 

 layer of the wall burst. //, 

 the last spore has not as yet 

 been extruded. ///, empty 

 ascus. After Pfeffer (Pflan- 

 zenphysiologie, ist edition). 



