72 DIVISION I. GENERAL MORPHOLOGY. 



one belonging to the sixth youngest spore in the chain. Phenomena essentially the 

 same occur in other species of the Uredineae, but with considerable variations in form 

 in the different species 1 . 



Where filiform sporophores rise free into the air, a further mechanical arrange- 

 ment is found which greatly assists the shedding and scattering of the abscised 

 spores. It may be readily observed in the Hyphomycetes, in Peronospora, for example, 

 Phytophthora infestans, and in the gonidiophores of Peziza Fuckeliana, &c. The 

 hyphae of these Fungi are cylindrical in the 'moist and turgescent state, but collapse 

 when dry and especially when the spores are ripe into a flat ribbon-like form 2 , 

 and the drier they are the more strongly do they become twisted round their own 

 longitudinal axis. They are so highly hygroscopic that the slightest change in the 

 humidity of the surrounding air, such for instance as may be caused by the breath 

 of the observer, at once produces changes in their turgescence and torsion; the 



latter give a twirling motion to the extremity of the 

 gonidiophore and the ripe spores are thereby thrown 

 off in every direction. 



Abjection of acrogenous propagative cells is effected 

 by a mechanism which we shall have to speak of again in 

 section XXI. The cell which is to be abjected, whether 

 spore or spore-mother-cell (for brevity we shall call it 

 spore), is abjointed singly by a cross septum at the apex 

 of a tubular and often comparatively large sporiferous 

 cell, a basidium or a sterigma, which retains its parietal 

 protoplasm still intact after the abjunction of the spore 

 and is still turgescent in consequence of a continued 



FIG. 38. Piioboius cedipus, Mont supply of water in increasing quantity. Its membrane 



Diagrammatical representation of a 



longitudinal section ; / upper end of is highly extensible and elastic, and continues to stretch 



the sporiferous cell, s the cell which 



will be abjected ; the transverse waii as the tension increases with the increased amount of 



which forms its boundary below thrusts 



its convexity into the interior of the water absorbed. But its cohesion is less over an annular 



cell ; r annular suture of dehiscence. 



area immediately beneath the cross septum than in 



any other part of the circumference, and if the tension reaches a certain point, 

 it overcomes the resistance of the less coherent annular zone, the suture of 

 dehiscence; the wall opens by a circular fissure, the pressure of turgescence is 

 instantly relieved and the elastic wall contracts, especially in the direction of the 

 transverse diameter, and this causes a large part of the fluid contents to be squirted 

 out at the same moment with force through the fissure, and as it strikes full on the 

 transverse septum, the spore that rests upon the septum is abjected with it. The 

 basidium thus emptied collapses and perishes. 



The process of abjection may be observed most completely in the acrogenously 

 abjointed spore-mother-cells of Pilobolus crystallinus and its nearest allies, of which 

 we shall speak again in later sections (Fig. 38). It occurs also, as Brefeld 3 has 



1 Bot. Ztg. 1. c. p. 786. De Bary, Brandpilze, p. 59. Reess, Rostpilzformen d. Coniferen, Halle, 

 1869. R. Hartig, Wichtige Krankh. d. Waldbaume, t. IV, V. 



2 Fresenius, Beitr. t. II. 



3 Bot. Ztg. 1870, p. 161. Abhandl. d. Naturf. Ges. zu Halle, Bd. XII, i. 1871. 



