PHENOMENA OF GROWTH. jgl 



PART FOURTH.— CHEMISTRY. 



LESSON XXXII. 



PHENOMENA OP GROWTH— CONSTITUENTS OF THE 

 PLANT. 



396. Active parts. 397, 398. Circulation. 399. Digestion. 400. 

 Formed Material. 401. Camphor-trees. 402. Inorganic Constitu- 

 ents. 403. Tabasheer. 404. Eaphides, Cystoliths. 405. Phospho- 

 rescent plants. 406, 407, Organic Constituents ; Pibrine ; 408. Cow- 

 tree. 409. Pood; 410,411. Poods and Poisons. 412. Saprolegnia. 



396. The active parts of an exogenous tree — which we 

 take as the model plant — are : 1 . The rootlets and their 

 fibrils, which are organs of absorption ; 2. The newest 

 wood, the newest bark, and the cambium-layer, which are 

 organs of circulation ; 3. The leaves, which are organs of 

 digestion. 



397. Circulation. — The cambium-layer is the medium of 

 communication between the wood and the bark. Its cells 

 are filled with a mucilaginous juice, called Crude sap, or 

 Pabulum (38), which is rich in protoplasm. 



In temperate climates it is most abundant in spring ; the oamhium- 

 cells are then so soft that the bark is easily separable from the wood. In 

 the tropics the cells maintain an even habit throughout the year. The 

 cambium-layer is the market-place of the plant, the great exchange, as 

 its name implies. On the side next the wood it deposits new wood- 

 fibres (Pig. 229, nw) ; on the side next the bark new bark fibres and 

 cells (I, g, co). Here, in both wood and bark, the laticiferous vessels 

 abound. Through these new cells, fibres, and vessels the sap circu- 

 lates, ascending and descending ; the ascending sap is the pabulum. 

 At the extremities of the roots, .stems, branches, and buds the cam- 

 bium is called primary. If a ring be chopped around a tree-trunk, 

 and deep enough to cut through the new bark, the cambium-layer, 

 I 14* 



