110 THE VEGETABLE WORLD. 
they lose a great part of their carbon, which passes into the state 
of carbonic acid, and they exhale large quantities of water. The 
result of these two phenomena is a decided elongation of the 
plant, a greater softness in its tissues, and the absence of green 
colouring. 
The juices of plants kept constantly in the dark modify them- 
selves in a sensible degree. Often acrid and bitter in their normal 
condition, they are rendered sweet and succulent under such modi- 
fications, and market gardeners turn these facts largely to their 
profit. They set Lettuce plants in artificial stations in order to 
make the hearts white, by binding them closely together and tying 
the leaves one against the other, to the exclusion of light. Sea 
Kale, which grows a wild, useless weed on the sea-shore, becomes @ 
delicate dish under the gardener’s care, aided by this principle of 
blanching. ' 
CIRCULATION. 
The manner in which nourishing juices circulate in the interior 
of plants has long been the subject of discussion amongst botanists, 
and science is still far from being agreed on this important point 
of vegetable physiology. Limiting ourselves, however, to the 
consideration of dicotyledonous vegetation, such as our indigenous 
forest trees, we may enunciate the following simple facts on which 
all botanists are agreed. 
Let us follow the juices of a dicotyledonous plant from the moment 
of their absorption by the root fibres. Let us see the course these 
liquids follow as they rise in the interior of the plant, and that which 
they take in order to descend again after having passed through the 
pervious tissues of the leaves, exposed to the chemical influence of 
the air; in other words, let us follow the progress of the sap both 
in its ascending and descending course. From the moment when the 
water which impregnates the earth has penetrated into the roots of 
a plant, and mingled its sap with the juices which are contained 
the cells of the vegetable, it constitutes what botanists call the sap, 
a complex fluid, which, at certain periods in the life of a plant, 
circulates constantly through its tubes. What is the force which 
causes the water to penetrate into the roots, impels the sap into the 
