Vegetable Physiology — Circulation of the Sap. 33 



their reach, and in the form of crude watery sap it ascends, mixed with the 

 substance stored up from the previous year, and lays the foundations for a 

 new layer of wood and bark. 



This great motion which we csdl flowing sap is quite different from the cir- 

 culation produced by the action of the foliage, and must not be mistaken for 

 it. It can be detected principally in early spring, weeks before the snow has 

 succumbed to the warm breath of southern winds, or the ice king has loos- 

 ened his grip of the soil. It can be detected slightly also in the autumn, in 

 some species, by cutting a branch or stem after a slight frost, yet, not dur- 

 ing the frost. Again, it has always been observed to flow from the sap-wood 

 and cambium layer of our trees, not from the heart-wood or bark. In early 

 spring, if a shoot of a grape-vine, birch or maple tree, be cut through with a 

 knife, they will yield a large amount of this fluid ; but the close observer 

 will notice even here that the upward flow is far more abundant than the 

 downward. Indeed, the flow from the dismembered shoot soon ceases, while 

 the flow from the wounded vine or tree continues for weeks, which proves 

 that this, flowing-sap is stored up in roots, stems, and branches, after the sea- 

 son's growth of the tree has ceased. 



We were taught in early youth that this flowing of the sap in early spring 

 demonstrated a continuous circulation, because, there being no leaves at the 

 time to carry it upward by exhalation, it was evident that if it were at these 

 periods running up the sap vessels, it must run down again by other chan- 

 nels. If this theory is correct, how is it, 



1. That this motion can no longer be detected when the leaves expand ? 



2. It moves only when a wound is made, being naturally at rest till the 

 leaves are developed; and, 



3. The effusion of sap from trees, when cut or wounded, is, during the 

 larger portion of the year, comparatively very small ; therefore, 



4. This flowing of the sap when the tree is wounded proves nothing more 

 than a facility of the sap to move, owing to the peculiar irritability of the 

 vegetiible body during the period of early spring. 



DOES SAP DESCEND? 



Fift}- years ago, our text-books on vegetable ])hysiology taught us that the 

 sap descended in the fall of the year with as much facility as it asccnietl in 

 the heat of summer. To prove this theory, the following illustrations were 

 given: " If a cord be tied tightly around the trunk of an exogen/xi o&Q\'s> 

 little impediment to the ascent of the sap, but will obstruct its diffusion 

 through the bark in its descent. In consequence, there will be a dcliciency 

 of nourishment to the parts beneath, and a superfluity above; so that a pro- 

 tuberance will arise from the stem just at the j)i)int where the downward 

 flow of the sap is checked. This protuberance will increase in progress of 

 years (if the tree survive) so as to bury the cord beneath it; but most com- 

 monly the tree is destroyed ere long by an insntlicicnt supply of nourish- 

 ment." 



