42 THE INNER ORGANIZATION OF TREES. 



year, the rings become confounded one with the other ; 

 they are much thinner, and so multiplied that they cease 

 to be reliable, and do not indicate in any manner the age 

 of the tree. 



The trees of temperate climates usually thicken them- 

 selves in their whole circumference, and the unequal de- 

 velopment of only one side must always be regarded as an 

 exceptional case. In tropical countries there are, however, 

 trees whose stems take the most wonderful forms, in con- 

 sequence of the unequal development of their sides. Some 

 of them belong to the genus Bauhinia, Natural Order, 

 Leguminosae. The most striking example among the spe- 

 cies of this genus is furnished by the stem of Heretiera 

 Fomes. The first year the stem of this tree is normally 

 formed, a small wood-ring surrounding the pith ; but after- 

 ward it annually thickens by crescent-like deposits of wood 

 on two opposite sides ; the stem thus presents a flattened, 

 compressed appearance. "I examined such a stem," says 

 Dr. Herman Schacht,* "which was eighteen inches in one 

 direction, in the other, on the contrary, it was only two 

 inches broad," presenting "in some measure, the appear- 

 ance of a natural plank, surrounded by a weak bark." 



Anomalous forms of Exogenous stems also exist amongst 

 tropical trees belonging to the Natural Orders Bignonia- 

 cese, Malpighiacese, Menispermacese, and Aristolochiaceae.f 



In the wood of beech-trees, two distinct species of cells 

 can be recognized, the fibre-cells and the vasiform or duct- 

 cells. 



The fibre-cells form the principal part of the wood of 

 each ligneous deposit. They are elongated and extremely 

 attenuated cells, tapering to either extremity, and lying to- 

 gether in bundles more or less compact, which are developed 

 vertically. It is through the fibre-cells of the wood that 

 the main current of the sap flows in the spring. Their vital 



* " Lehrbuch der Anatomie und Physiologie der Gewachse," page 345, 

 Berlin, 1856. 



f See, "Precis de Botanique et de Physiologie Vegetale," par A. Rich- 

 ard j page 75-80, Paris, 1852. 



