CHAP. XIII. 
CIRCULATION OF THE SAP. 
395 
peripherical vascular system of animals. The vessels con- 
tract and become so small as to be invisible, they then fill 
themselves again, enlarge, and re-establish the communica- 
tion which had been interrupted. It often happens that 
when strong currents are formed, the weak ones disappear. 
If a current is about to stop, it may be seen to oscillate a 
moment both in front and rear. If the globules are amassed 
in a particular place, an obstruction takes place, and the 
fluid part of the latex is no longer capable of passing along. 
If we take a thin slice of bark, or better still, certain en- 
tire organs, very thin, transparent, and young, but fully 
formed, in which the latex has an abundance of globules, 
it is often easy to observe a translation of fluid, and to appre- 
ciate its rate of motion, by the time which the globules take 
in moving a certain space. In cases where the motion of 
cyclosis cannot be actually seen in the vessels, it may be in- 
ferred from the following fact. When the two ends of a 
stem containing milk are cut through, the latex is seen to 
run out at both ends of the fragment, which proves that there 
must be both an ascending and descending current : the same 
phenomenon is visible in plants having a colourless latex; 
therefore there must be a motion of ascent and descent in 
them also. 
2. It occurs in the greater part of monocotyledonous and 
dicotyledonous plants, and the vessels in which it takes place, 
are so generally in connection with spiral vessels, that the 
presence or absence of the one is usually accompanied by 
that of the other. The situation of the vessels in which it is 
found is, in the root, stem, petiole, peduncle, flower, &c. 
The system of vessels, in the form of a delicate network, 
surrounds the cells, and even traverses their interior, in the 
most diverse directions. In the stems of monocotyledons, 
cyclosis occurs in the woody bundles, as also in those dico- 
tyledons which have their wood in like manner separated 
into distinct cords. But, in the stems of dicotyledons where 
the wood is disposed concentrically, the vessels of the latex 
are either placed singly, in the parenchyma of the bark; 
or, which is most common, they either form a continuous 
envelope around the wood, or bundles arranged circularly, or 
