262 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



favourable circumstances, may again develop further, whilst the emptied 

 upper portion dies away. Vaucheria geminata 1 behaves in quite the 

 same way, only the breaking up into single cells takes place within 

 the green tube of the plant ; it divides into a number of thick-walled 

 cysts which protect the protoplasm better against drought than would be 

 the case were the threads merely to pass over into a resting condition 

 without becoming divided. 



This construction corresponds with the formation of the sclerotium 

 of the Myxomycetes, the building of which is likewise associated with 

 an important change in configuration of the vegetative body. These 

 sclerotia are resting conditions of the mature plasmodia 2 , and when 

 they are being formed the delicate processes of the plasmodium are 

 drawn in and it breaks up into cells, invested by a membrane, which 

 are capable of withstanding drought. This resting condition appears 

 as a consequence of prolonged drought, but it seems also to be a conse- 

 quence of unfavourable nutrition as well as of other unfavourable influences. 

 A similar resting condition from similar causes may be assumed by 

 the swarm-cells or amoebae and the juvenile plasmodia ; they become 

 thick-walled cysts. 



The protonema of some mosses has the same capacity and under 

 unfavourable circumstances becomes divided into isolated cells. Thus in 

 Funaria hygrometrica colourless ' limiting ' cells whose walls become partly 

 mucilaginous appear between the ordinary green protoplasmic cells and 

 bring about a separation of these cells which remain alive 3 . The 

 biological significance of these processes is chiefly to be found in the fact 

 that the isolated cells can more easily again attain to favourable conditions. 



In the higher plants the appearance of resting conditions, which 

 correspond biologically with those of the plasmodia, are in most cases 

 independent of external influences, in gardening practice however an 

 earlier commencement of the resting period is brought about by a 

 slackening of the water-supply in plants which are destined to be 

 ' forced.' 



C. FORMATION OF TUBERS IN JUNCUS AND IN POA BULBOSA. 



The formation of tubers in Juncus supinus and other species 

 requires further experimental and anatomical examination. According 



1 Stahl, tiber die Ruhezustande der Vaucheria geminata, in Botan. Zeitung, 1879, p. 129. In 

 Oedocladium protonema, referred to in the text on p. 256, resting stages arise which are able to resist 

 long desiccation, but such states also develop on the subterranean parts of plants growing normally ; 

 see Stahl in Pringsh. Jahrb. xxiii. p. 343. 



2 See cle Bary, Die Mycetozoen, p. 460 ; Cienkowski, Das Plasmodium, in Pringsh. Jahrb. iii. 

 p. 422. 



3 Goebel, Die Muscineen, p. 389; Id. Uber Jugendformen von Pflanzen und deren kiinstliche 

 wiederhervorrufung, in Sitzungsber. der. k. bayer. Akad. d. Wissensch. xxvi ^1896), p. 456. 



