272 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Resting-spores of Algse.* — N. Wille describes the mode of 

 formation of tlie non-sexual reproductive cells, commonly known as 

 resting-spores, in a number of filamentous algse, as Trentepolilia 

 (Gongrosird) de Baryana, Conferva fachy derma, C. stagnorum, G. 

 WittrocJcii, . C. homhycina, TJlothrix Pringsheimii, &c. All these cases 

 agree in the reproductive cells thus formed being immotile, not 

 produced by any sexual process, and not resulting from swarm-cells 

 that have come to rest. They may, however, be divided into two 

 classes, those produced without any special cell-formation, as in the 

 cases of TJlothrix, Conferva pachyderma, and TrentepoJilia, or after special 

 cell-formation, as in Conferva stagnorum, C. Wittrockii, C. homhycina, 

 and Pithophora. The former kind the author proposes to call 

 Alcinetes, the latter Aplanospores. Both kinds vary in this respect, that 

 they may germinate immediately after their formation, or only after a 

 period of rest. In the former case they perform the function of 

 zoospores, i. e. increasing the number of individuals ; in the latter case 

 they act like zygotes. 



In Conferva, TJlotlirix, and (Edogonium, the mode of formation of 

 the resting-cells resembles that in the Conjugatae. The membranes 

 of the filament become thicker, and incrusted with iron and lime ; as 

 soon as the separate cells again begin to grow, the outer dead layer 

 bursts, and the form arises described earlier as a distinct genus under 

 the name Psichohormium. In Conferva pachyderma, the akinetes are 

 formed by a stronger deposition of cellulose in the inner cell -wall 

 layer, while the outer ones become mucilaginous, and the separate 

 cells are thus set free. The step to the formation of aplanospores in 

 C. stagnorum and WittrocMi is a very short one. In Cladophora 

 fracta single cells at the ends of filaments often swell up in the 

 autumn, and become thicker walled and fuller of protoplasm. These 

 hibernate, filaments with thin-walled cells springing from them in the 

 spring. A similar process takes place in Conferva homhycina and in 

 Pithophora. In Trentepohlia de Baryana two kinds of akinetes are 

 formed. 



No exact boundary line can always be drawn between akinetes 

 and ordinary vegetative cells ; aplanospores differ more widely from 

 the latter, but pass insensibly into the former. The author's view 

 is that these structures are formed whenever the conditions are un- 

 favourable for the formation of zoospores or for a sexual mode of 

 reproduction. Where they are abundantly produced, it is usual for 

 the formation of zoospores to be rare. This is the case in Conferva 

 stagnorum, and Cladophora fracta, while in Conferva pachyderma, 

 Wittrockii, and homhycina, TJlothrix Pringsheimii, and Pithophora they 

 are at present unknown ; in most species of Cladophora they are abun- 

 dant. In Trentepohlia umbrina, quantities of swarm-cells are formed, 

 but they rarely either conjugate or germinate ; in T. de Baryana they 

 are also formed, but soon perish, reproduction taking place by means 

 of akinetes. 



The author regards the resting-cells as affording good characters 



* Bot. Gesell. Stockholm, Sept. 26, 1883. See Bot. Oeutralbl., xvi. (1883) 

 pp. 215-9. 



