D. B. WARD, M. D. 71 



plants forming the green scums which mantle the surfaces 

 of our fresh water ponds and ditches, and which we 

 know as frog spawns. They are very humble plants, well 

 down toward the lowliest of organisms. I have already 

 alluded to the motion of the free-swimming kinds. By 

 free swimming I do not mean that they are simply unat- 

 tached and drifting about at the mercy of small currents 

 of water, but they actually progress in the direction al- 

 ways of their long axis and with considerable speed in 

 some cases. I have seen specimens of pleurosigma, for 

 example, moving about as fast as the Mary Powell does 

 in proportion to their size, i. e. the pleurosigma would 

 move through its own length in about the same time that 

 the steamer would require to traverse her own length. 

 So you see the motion is rapid in some cases. Now 

 what is the cause of this motion? I don't know, and 

 the worst of the matter is no one else does. There has 

 been no end of theories about it but none which explains 

 ail the facts. There is something weird and ghost-like 

 about it, as they leave no commotion in the water behind 

 them, nor any disturbance of minute particles in their 

 vicinity. I have seen a navicula run into a little mass 

 of decayed vegetable matter and back out, then change 

 the direction of its axis somewhat and proceed again, 

 clearing the obstacle. I remember a good while ago see- 

 ing a little crab in an aquarium butting against the glass 

 time after time in an effort to go in that particular di- 

 rection. He kept at it until I Avas tired of watching 

 him, and when I saw this diatom I thought that to all 

 appearance the diatom was more intelligent than the ob- 

 stinate little crab, although the latter was a highly organ- 

 ized animal and the. former one of the lowliest of plants. 

 I have no doubt that the change irt direction of the axis 

 just mentioned was accidental in this case but the back- 

 ing away from the vegetable debris was not accidental, 

 but wa§ similar to the action of the sensitive plant in 



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