CELLULAR TISSUE. 23 



are removed in any part, tlie current is arrested at tliis point 

 and the floating globules accumulate there, until finally they 

 are deflected from their course and return by the o|)posite 

 current, as seen at tig. 8, a. These phenomena occur in per- 

 tectioQ only in the young internodes. As the parts become 

 old, the globules become detached in spots, the current becomes 

 irregular in proportion. In more advanced age they often 

 become entirely removed iVom tiic surface of the cell and float 

 in the contained fluid, which ceases to circulate. At other 

 times they entirely disappear. 



*21. Any cause, which will accelerate or retard vegetation, 

 accelerates or retards this circulation. Within certain limits 

 heat will accelerate the movement, and cold retard it. Ex- 

 cess of either will destroy it entirely, as it does the life of tho 

 plant. Light and atmospheric air are necessary for its con- 

 tinued motion. Poisons act variously on the circulation and 

 the motion of the intercellular fluid is a true index of its ef- 

 fect, as its change is the first indication of their influence. 

 This plant has been made the means of determining what 

 substances are poisonous and their mode of action, and is said 

 to be the most delicate test for a poisonous substance, and is 

 called by Raspail a Toxicometre. 



From these observations it is evident that the propelling 

 power resides in the green globules. It is by no means neces- 

 sary, however, that the globules should be green, as the same 

 arrangement is tbund in the roots and the same circulation 

 takes place, and the globules are white. To observe this cir- 

 culation two conditions are absolutely necessary. First, that 

 the vessicle should be transparent. Second, that it should 

 have globules floating in it by which its movements may be 

 detected. Should the walls of the cell be transparent with a 

 uniformly dense fluid circulating within it, it would be impossi- 

 ble to distinguish its motion. 



22. To discover the immediate cause of this circulation 

 has exercised the ingenuity of many philosophers. Amici 

 advanced the idea that the motion of the fluid in the cells was 

 owing to an impulse given by the young globules, acting as 

 a galvanic pile ; but M. Becquerel and Dutrochet made vari- 

 ous experiments on the subject, and from the result concluded 

 that the force, which produced the rotary motion of the fluid 

 within the vessicle, is not electricity. Electricity, by the bat- 

 tery, would suspend the movement when of sufficient intensity, 

 but exerted no other influence on it. This ellect was with- 

 out doubt mechanical, as it made no diflerence how the poles 

 were arranged in reference to the rotatory movement of the 



