INTRODUCTION TO CRYPTOGAMIC BOTANY. 429 



Agents destructive to life, if sufficiently energetic, arrest the 

 circulation ; if not powerful enough to destroy it altogether, 

 they merely arrest it for a time till the equilibrium is restored. 

 This motion is accelerated by heat within certain limits, and 

 beyond these it is gradually diminished; it is again accelerated, 

 and at 113°, Fahr., it ceases altogether. Electricity, in a simi- 

 lar manner, produces temporary arrests of motion, but it does 

 not seem to cause acceleration. Brongniart and Dutrochet 

 saw similar effects produced by pressure. Dutrochet after- 

 wards placed a Chara under the influence of a powerful elec- 

 tro-magnet. It had, however, no effect on it whatever. 

 Neither magnetism nor electricity, then, are the causes of 

 the motion. The first has no influence on the motion, the 

 other acts merely as any other exciting cause, and the vital 

 action must be considered as something sui generis* Various 

 bodies have been observed by Goeppert, Cohn, and Carter, in 

 the circulating mass, whose nature is not perfectly ascertained. 

 All are probably not of the same nature. Some are appa- 

 rently gonidia, and others infusoria. Some of these bodies, 

 according to the former authors, are densely clothed with 

 vibrating cilia, and something of the same kind appears to have 

 been observed by Carter. For particulars I must refer to the 

 papers quoted below. "f" 



473. Many of the species emit an intolerable smell of sul- 

 phuretted hydrogen, which is supposed to cause fevers in some 

 districts, as in the Pontine marshes. I have, however, known 

 a whole district pervaded by this smell, for many days toge- 

 ther in very hot weather, without producing the slightest bad 

 effects. It has been suggested that they may be useful as 

 manure, from their containing carbonate and phosphate of 

 lime. 



474. OharacecB, like other Cryptogams, are subject to great 

 variations, which make the species difiicult of determination. 

 Not only does the size, the degree of ramification, the produc- 



* See Comptes Eendus, Ap. 15, 1846, and Ann. of Nat. Hist., vol. 



IV, p. 451. 



t Bot. Zeit. 1849, p. 665, &e.; Ann. of Nat. Hist., n. s., vol. 17, 



p. 101. 



