ON CAMBIUM. 85 



wood are the medium through which the sap both as- 

 cends and descends ; but you may recollect that it is not 

 well ascertained, whether the ascending sap rises through 

 the vascular or cellular system, or through the interstices 

 between them ; and the v-essels which convey the de- 

 scending sap being so minute as barely to be discernible 

 by the aid of a microscope, it is impossible to examine 

 them with accuracy. Besides which a still greater diffi- 

 culty attaches to the investigation of the vessels of endo- 

 genous plants : those which grow in our climates being 

 too small to enable them to acquire that degree of vigor 

 which is requisite for a complete developement of their 

 organs. 



Caroline. We shall not have the same difficulty to 

 account for the descent of the cambium, as we have had 

 for the ascent of the sap ; since it obeys the laws of 

 gravity and descends by its own weight. 



Mrs. B. That is a general cause of the descent of 

 cambium, no doubt ; but in the weeping-willow, and 

 many other trees whose branches are pending, some ad- 

 ditional cause is required to produce the motion of the 

 cambium, since it must rise to return into the stem. It 

 has been ascertained that agitation facilitates and accele- 

 rates this motion, and consequently increases the vigor 

 of vegetation ; for the more rapidly this nutritive fluid 

 circulates through the several organs, the more frequent- 

 ly it will deposit its nutritive particles in them. Mr. 

 Knight has made a variety of interesting experiments on 

 this subject. He confined both the stem and branches 

 of a tree, in such a manner that it could not be moved 

 by the wind. The plant became feeble, and its growth 

 much inferior to that of a similar tree, growing in a nat- 

 ural state. Mr. Knight confined another tree, so that it 

 could be moved only by the north and south winds, and 

 obtained the singular result of an oval stem ; the sides 

 accessible to the wind growing more vigorously than 



461. What is probable 1 ? 462. Wha* further difficulty attaches 

 to the investigation of the vessels of endogenous plants'? 463. What 

 is a general cause of the descent of cambium'? 464. In what trees 

 must there be some other cause 1 ? 465. What has been ascertained to 

 produce an accelerated motion of cambium, and consequently an increas- 

 ed vigor in vegetation'? 466. What was the first experiment made 

 by Mr. Knight with its result, in relation to this subject 1 467. 

 What was the second one with its resultl 

 8 



