290 LETTER XXV. 



gravely and slowly, retreating in like manner, altering 

 their direction, and finally possessing a spontaneous, 

 incessant, measured, voluntary motion. These little 

 creatures, which at most are not more than the 1000th 

 of an inch long, and at the smallest hardly exceed the 

 5000th, when once they are separated from the thread 

 that contains them, fall down in countless multitudes, 

 in the form of a chocolate-brown deposit, on the 

 neighbouring rocks. Once there, they distend, and 

 emit a globule of coloured particles, which are evi- 

 dently their fry. Each particle gains motion and 

 volume, and the little globular mass, lengthening and 

 branching, reproduces, by the developement of the 

 germs that are collected together, the long green pen- 

 cilled appearance, which has led Botanists to consider 

 this being as a plant. 



In another production, green ditch-Laver (Ulva 

 buUata, or minima, or Tetraspora lubrica), still more 

 astonishing circumstances have been observed by M. 

 Gaillon and others. This plant appears, to the naked 

 eye, a thin green membrane, within which the mi- 

 croscope reveals a number of green granules, arranged 

 in fours. Let this membrane be kept in quiet water, 

 and at a high atmospheric temperature, and the 

 granules may be seen, under a powerful microscope, 

 to present at their surface certain convexities and 

 depressions, which are the effect of the repeated con- 

 traction and distension of these granules. If they are 

 carefully watched for several days, the granules will 

 be seen to be reciprocally displaced ; after a certain 

 time they separate from the membrane, and may 



