120 OYOLOSB. 



floating m it. The roots of the plant also are of the same con- 

 struction, and contain the same kind of fluid, suspending like 

 globules. The tubes of the stem are lined on their inside with 

 innumerable green elliptical globules placed end to end. By re- 

 moving the cortical tubes with care and applying the micro- 

 scope, we observe the floating globules following with perfect 

 regularity the direction of the spirally arranged globules attached 

 to the tube. The ascending current, when it arrives near the 

 node, turns and forms a descending current on the opposite side, 

 following with equal regularity the green globules. Between 

 these two currents there is a line destitute of green globules, and 

 under which the fluid does not circulate, and which is called the 

 line of repose. If the green globules make accidentally any 

 sinuosities, the floating globules follow these sinuosities. If the 

 green globules are removed in any part, the current is arrested 

 at this point, and the floating globules accumulate there, until 

 finally they are deflected from their course and return by the 

 opposite current. These phenomena occur in perfection only in 

 the young internodes. As the parts become old, the globules 

 become detached in spots, and the current becomes irregular in 

 proportion. In more advanced age they often become entirely 

 removed from the surface of the cell and float in the contained 

 fluid, which ceases to circulate. At other times they entirely 

 disappear. 



232. 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. Excess 

 of either will destroy'it entirely, as it does the life of the plant. 

 Light and atmospheric air are necessary for its continued mo- 

 tion. Poisons act variously on the circulation, and the motion 

 of the intercellular fluid is a true index of its effect, 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 Toxicometer. 



233. The power which plants possess of accumulating sap, 

 and drawing on this store, as food for future use, is a subject of 

 much interest, and of much practical importance. Striking ex- 

 amples of this kind we see exhibited in the Radish, Turnip, Beet, 

 &c. In these cases the energies of the plant are spent, in the 

 first period of their existence, in laying in stores of food in the 



How docs the current rr.ove? — 232. Effect of external agents? How dc 

 poisons affect it? What does Raspail call it?— 233. What is said of the 

 accumulation of sap? Examples. Explain tb em. 



