568 Comparative Physiology. 



to elongation of the cells in the direction in which fluid is con- 

 veyed. 



" In Phanerogamia, the annual layer consists of dotted ducts 

 and woody fibre, mixed with cellular tissue ; the vessels at the 

 inner side of the ring, and the fibres external. In long slender 

 stems requiring rapid circulation, as the vine, and in dense stems, 

 as oak, mahogany, 8fc., which do not transmit so readily, the 

 vessels are largest in diameter. In the bark the cellular matter 

 predominates over the fibro-vascular ; there are also more in- 

 tercellular spaces or passages than in the stem, as well as those 

 branching and anastomosing tubes, which appear destined to the 

 conveyance of elaborated sap, termed laticiferous vessels. The 

 footstalk of each leaf is connected with the wood and bark, the 

 upper stratum of vessels in the leaf and stalk being connected 

 with the wood, and the inferior with the bark. These surfaces 

 remain distinct while the leaf continues, their functions being 

 importantly different. 



" The course taken by the sap is the following. The fluid ab- 

 sorbed by the roots is conveyed upwards through the stem, by 

 the woody fibre and ducts of the alburnum, to the upper surface 

 of the leaf. Much of the watery portion is then exhaled, and 

 an interchange of gaseous ingredients takes place with the at- 

 mosphere, by which a large quantity of carbon is added. The 

 fluid transmitted along the inferior stratum of vessels to the 

 bark contains the peculiar secretions of the plant, and is adajDted 

 to supply the demands of its nutritive functions. This fluid, 

 now termed elaborated sap, jDi'oper juice, or latex, descends 

 through the cellular tissue and intercellular passages of the 

 bark, furnisliing the materials of the new layers which are being 

 added to the alburnum and inner bark ; and a portion is carried 

 to the interior of the stem by the medullary rays. Very little 

 reaches the roots, and none, unless the small quantity which 

 mixes with the ascending sap, is again sent through the system. 



" The movement of the elaborated sap in its jii-oper vessels 

 has recently been made the subject of much careful observation. 

 Schultz, who first noticed it in plants with milky juices, thought 

 it peculiar to them ; but there is now good reason to believe 

 that it is common to all vascular plants. The channels are not 

 straight tubes like the ducts in which the sap ascends, but of 

 irregular shape, slender, and inosculate freely with one another 

 like the capillaries of animals. They are arranged like the net- 

 work of passages in many of the lower animals, as Planarias, 

 which have no central organ of impulsion. The movement had 

 been termed cyclosis, to distinguish it from the rotatory nu- 

 tritive movement observed in single cells. It may be observed 

 in thin slices of the bark under the microscope, the stipules of 

 i^cus elastica, the leaves and valves of the fruit of Chelidonium 



