The Plant Cell 31 



have lost the cytoplasmic strands. It may be observed 

 in the water weed Elodea, and attains a maximum rate 

 and clearness in an inner parietal cytoplasmic layer of 

 the internodal and other ccenocytic segments of the alga 

 Nitella (stonewort). In this last-named plant it is not 

 uncommon to find, at a temperature of 28 to 30 C., a 

 rate of movement from 3 to 4 mm. per minute. 



Movements of orientation result in a gradual, or scarcely 

 directly visible, shifting of a portion of the cytoplasm or of 

 other portions of the protoplast. By this means the nu- 

 cleus is able to change its position in the cell, and the plas- 

 tids (chloroplasts) show peculiarities of arrangement under 

 different intensities of light. Orientation is doubtless to 

 a considerable extent characteristic of all living cells, but 

 the result of the movement is more easily noted in those 

 green cells quickly responsive to changes of light. 



24. Protoplasmic irritability and response. The move- 

 ments previously referred to are indicative of a type of 

 activity. It is of interest to note to what extent this 

 activity may be affected by a change in the environment, 

 as, for instance, a change in temperature. If favorable 

 material of any kind (such as Nitella, Tradescantia, 

 Cucurbita) is carefully studied at different temperatures, 

 effected by a temperature stage, it will be found that with 

 respect to rate of movement there is, in general, a mini- 

 mum, an optimum, and a maximum temperature for move- 

 ment, so that the protoplasm is highly responsive to these 

 differences of the environment. 



This change in rate of motion with the above-mentioned 

 minimum-optimum-maximum manifestation is obviously 

 an indirect effect. Moreover, since temperature changes 



