52 ON STEMS. 



Mrs. B. No ; they increase throughout their whole 

 length. If you make marks at certain distances on a 

 root, you will find that these distances are not altered by 

 growth ; but if you make similar marks on a stem or a 

 branch, the distances will increase, showing that it grows 

 in its whole extent. 



Emily. It must be so, since a new layer of wood grows 

 annually at the base. 



And pray, through what part of the stem does the sap 

 rise. 



Mrs. B. That is a question which has been long and 

 much disputed. Some naturalists have maintained the 

 opinion that it ascended through the pith : others, that 

 it rose through the bark: and they have reciprocally 

 proved each other to be mistaken in their conjectures. 

 A third road was, therefore, sought for ; and, by coloring 

 the water with which a plant was watered, it has been 

 traced within the stem, and found to ascend almost wholly 

 in the alburnum or young wood, and particularly in the 

 latest layers. 



Caroline. That is very natural. The perfect wood 

 has in a manner finished its active career : it can itself 

 acquire but little nourishment ; and its indurated texture 

 would be ill adapted to the conveyance of the sap, whilst 

 the young layers being in the full vigor of growth, and 

 their cellular system flexible and elastic, are much better 

 calculated to transmit it ; besides, it is in these, you say, 

 that the 1 young shoots take their origin. 



Mrs. B. The sap does not impart nourishment to the 

 plant during its ascent : it is therefore more probable that 

 its rising through the new wood is owing to that being 

 softest and most permeable. By means of the colored 

 medium I have mentioned, it was observed that the sap 

 naturally ascended in straight lines, but that, if it encoun- 

 tered any obstacle, it could move obliquely, or even 

 spread itself laterally. 



261. Do the sterns and branches of exogenous plants grow like their 

 roots, merely at their extremities'? 262. What two opinions prevailed 

 with naturalists as to the part of a stem in which the sap ascends 1 ? 263. 

 How has it since been ascertained to ascend through the alburnum! 264. 

 Is nourishment imparted by the sap in its ascentl 265. How did the 

 sap naturally ascend as it appeared from the experiment with colored 

 water! 



