206 PKOTOPLASM. 



568. On the other hand, the protoplasm of dry seeds can be 

 subjected to extremely low temperatures without suffering anj' 

 injury (see "Germination"). 



569. Tlie relations of protoplasm to light are best examined in 

 the Plasmodia of the mj^xomycetes and the hairs of Tradescantia, 

 for here they are not complicated by the presence of chlorophyll 

 (which, as will be seen later, exerts a marked influence). Ac- 

 cording to Hofmeister, plasmodia thrust forth longer and more 

 numerous processes in darkness than in light. In ^thaliiim sep- 

 ticum the processes developed in light are short and compressed, 

 while those grown in darkness are long, slender, and thin.^ 

 This is especially noticeable when the light falls only on one 

 side of the mass. In some of Baranetzky's experiments,^ in 

 which the incident rays of light were parallel to the substratum 

 (wet filtering-paper) on which the Plasmodium was placed, the 

 change of form resulting from diminished extension on the 

 lighted side and increased extension on the other was very 

 marked after fifteen minutes' exposure to bright sunlight, while 

 in diffused light half an hour was required for a similar change. 

 These results should be compared with those obtained by 

 Schleicher,' who observed that young plasmodia move towards 

 light of low intensity, and that older plasmodia ma}' move even 

 towards strong light. The movement into bright light appears 

 to just precede the formation of the spores. 



570. The more refrangible rays of light — that is, the violet 

 and indigo — appear to be more efficient in influencing move- 

 ment than are the less refrangible, — the red and j-ellow. 



571. The " circulation" of protoplasm in plant-hairs goes on 

 not only in darkness, but even when the hairs are developed on 

 plants blanched by absence of light.^ No marked effect upon 

 the rate of such movement appears to be caused by presence or 

 absence of light, except so far as the concomitant action of heat 

 comes into play. Hofmeister states that he saw the protoplasmic 



1 Die Lelire von der Pflanzenzelle, 1867, p. 21. 



2 Memoires de la soc. des sciences nat. de Clierbourg, 1875, p. 340. It is, 

 however, well known that plasmodia often emerge slowly from their sub- 

 stratum ; for instance, tan, if the surface is only very faintly lighted. 



3 Jenaisehe Zeltschrift, 1878, p. 620. 



* Sachs : Botan. Zeit., 1863, Supplement. Eeinke : ibid., 1871, p. 797. 

 Kraus : ibid., 1876, p. 504. Few observations have been recorded upon the 

 effect upon protoplasmic movements of sudden changes of illumination. In 

 the case of an amceba (Pelomyxa palustris) Engelraann found that light, 

 and not its sudden withdrawal, appeared to exert a stimulant effect (Pfeffer : 

 Pflanzenphysiologie, ii. p. 387). 



