IRRITABILITY 219 



callj' affects substances contained in the cell, and that the 

 changes in these stimulate the protoplasm (see p. 210). 

 The protoplasm of one or more cell-members of a tissue may 

 react by changing the form, size, turgor, or some other 

 quality of the cells, thus causing a change in the form or 

 direction of an organ ( heliotropism ) . This change may be 

 made permanent by growth without being directly accom- 

 plished by growth. The same change in form, size, turgor, 

 or other quality of isolated cells may result in locomotion 

 (heliotaxis). In both instances the effect of light might be 

 intrinsically the same, the reaction of the protoplasm to 

 chemical stimulation also the same, the result being different 

 only because one cell could move freely while the other was 

 obliged to move more than itself.* 



INFLUENCE OF HEAT 



The degree of heat prevailing in the medium — air, water, 

 soil — in which a plant is growing, and the radiant heat fall- 

 ing upon the plant and its immediate surroundings, exert 

 definite influences upon its activities. Without its minimum 

 degree of warmth, the plant will survive only in a resting 

 condition, if at all. There is also a maximum temperature 

 which can be withstood only in the resting condition. Ap- 

 parently there is no temperature so low as to be fatal to 

 resting protoplasm — seeds, spores, etc. — and the maximum 

 for dry resting protoplasm is surprisingly high. There are, 

 however, minimum, optimum, and maximum temperatures 

 for active protoplasm, which are definite and definitely re- 

 lated to each other. Organisms living where the mean 

 temperature is low have low minimum and maximum tem- 

 peratures. The native plants of warm situations have cor- 

 respondingly high extreme temperatures, and plants accus- 

 tomed to living where there is only slight variation in 

 temperature can be cultivated only under similar con- 



* Loeb, J. Zur Theorie der physiologischen Licht und Schwerkraft- 

 wirkungen. Archiv f. d. gesam. Physiologie, Bd. 66, 1897. Einleitung in 

 die vergleichende Gehirnphysiologie und vergleichende Psychologie. Leipzig, 

 1899. 



