upon the Distribution of Vegetables. 173 



in those of the Hydrocharis. The roots are, besides, co- 

 vered with a layer of epidermis, differing from the epidermis 

 of the stem, in the circumstances of its cellules having no 

 particular form, and in being destitute of stomata. When ex- 

 posed to the contact of the air, this layer thickens, becomes 

 tough, and covers the root with an envelope like parchment, 

 as may be observed in the aerial roots of Pothos and Epiden- 

 drum. I have examined in the Lemna, a very important or- 

 gan of the root, namely, the coleorhiza. The root of Lemna 

 is covered with a double layer of epidermis ; and so long as 

 the root is not more than a line and a half long, the two lay- 

 ers are combined ; but when it lengthens, the outer layer is 

 ruptured horizontally, and remains united to the inner one 

 only towards the point of the root. We can perceive traces 

 of it at the base of the root, although they are very indistinct. 

 If we cut off the extremities of such roots as are furnished 

 with a coleorhiza, the plant is not destroyed ; for new radicles 

 are instantly developed, and, in about ten days, attain the 

 same length as the first. It is certain that the epidermis, and 

 even some isolated layers of the bark of the roots, are sepa- 

 rated from the cellular tissue which they cover, and are de- 

 tached, in the same manner as we frequently observe in the 

 trunk. Some authors have even supposed that they fall like 

 the leaves. Without adopting so decided an opinion, we will 

 readily admit that these organs are in great part renewed. 



The respiratory functions of the leaves are connected with 

 the absorbing properties of the root ; they increase and dimi- 

 nish together. The leaves absorb by means of their hairs, 

 which are precisely analogous to those of the roots. The 

 leaves of plants exposed to the sun, become either villous or 

 tough ; the plant by this means restores the equilibrium, and 

 replaces the loss occasioned by a too rapid evaporation. This 

 explanation is different from that of M. DeCandolle, who 

 thinks that the hairs, in closing the openings of the stomata, 

 prevent a too abundant evaporation ; but the definitive result 

 is the same. The following experiment proves that the roots, 

 in the absence of the green parts, may supply their place. — 

 I raised from seed a Solanum Lycopersicum ; the cotyledons 

 were cut off as soon as they had performed their office, and 

 as fast as the leaves appeared, they also were cut off. Adven- 

 titious roots were soon developed at the base of the stem, 

 which became more and more numerous ; the plant lived and 

 flowered, but produced no fruit. It is not yet well ascer- 

 tained, whether plants merely assimilate the constituent 

 principles of the air, or whether they are able, of them- 

 selves, to produce carbon. The experiments of Crell seem to 



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