42 THE INNER ORGANIZATION OF TREES. 



ing warmer climates, where vegetation continues almost 

 without interruption throughout the whole year, the rings 

 become confounded one with the other; they are much thin- 

 ner, 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 themselves 

 in their whole circumference, and the unequal development 

 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 consequence of the 

 unequal development of their sides. Some of them belong 

 to the genus Bauhinia, Natural Order, Leguminosa3. The 

 most striking example among the species 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 afterward it annually thickens 

 by crescentlike 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 appearance of a natural plank, surrounded by 

 a weak bark." 



Anomalous forms of Exogenous stems also exist amongst 

 tropical trees belonging to the Natural Orders Bignoniacea3, 

 Malpighiacea3, MenispermaceaB, 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 



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

 Berlin, 1856. 



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

 ard; page 7580, Paris, 1852. 



