EXTRACTS—NATURAL HISTORY. 
137 
preserved. Should, however, the roots come to furnish more, and the leaves 
evaporate less, then will ensue turgescence in the vegetable tissue; and if a hole 
be made, the sap or liquid will overflow. This is precisely what is observed in 
the birch tree, in spring, when it begins to rise, and before its leaves have come 
forth or are able to perform their task of evaporation. As another trait of re- 
semblance, it may be remarked, that the lateral action of heat on any hygros¬ 
copic column, such as we have represented the vegetable tissue to be, would have 
the effects of rendering it capable of less saturation, and consequently, would 
oblige it to throw out a part of the liquid it contains. This is the effect which 
the sun produces upon the birch, and upon other trees, whose sap runs out at 
this period. When the leaves come, these phenomena cease; the task of evapo¬ 
ration is performed, and the sap bursts neither from the bark nor through am 
orifice, if made. Now suppose we replace the impermeable or air tight envelope 
by one, on the contrary, capable of absorption from within, and exhalation from 
without, the state of things will be changed. The issue of the sap or liquid by 
the sides of the envelope will be more frequent and fac'le. The diminution of 
the exhaling power by a sudden cold will favour it, and the sap will burst forth 
at once from all the pores of the tree equally, taking into account merely the 
different degrees of thickness in the bark. Such is an account of the emission 
of sap by the sides of the nut tree and sycamore, in spring. 
The influence of the leaves on the eternal motions of the sap in trees being 
thus explained, let us observe what will be the consequence, if these leaves, or 
great evaporating organs, be enveloped with a colder atmosphere. The sap 
conveyed to them being no longer evaporated, will rest and collect on their sur¬ 
face, and check all evaporation, especialty at night. The upper parts of the 
vegetable tissue, or hygroscopic column, being thus overcharged, will let fall 
their superabundance upon the parts that are lowest, which will produce a 
descent of the sap. Henoe proceed the alternate ascent and descent of the sap, 
such as have been noticed. Moreover, these effects will become continuous, 
if the evaporating property of the leaves should diminish before the supplying 
power of the root ceases to throw up the sap : and this is precisely the case in 
September: the same trees that afforded but their ascending sap in the spring, 
in September afforded a continual sweat. The latter was no longer the same as 
the spring sap, for it contained no saccharine principle. 
Mr. Boit concludes, from his experiments, that the alimentation of the 
foliaceous organs is accomplished principally during the day, whilst the alimen¬ 
tation of roots, and the formation of new layers of them, is effected during the 
night, when the diminution of evaporating power in the leaves precipitates the 
sap in a descending course towards the roots. 
That in deciduous trees, the annual increase of the trunk and branches takes 
place in winter. The ascending motion is thus suspended by cold, and the 
absence of leaves allows the sap to accumulate in the roots, which experience 
little of the atmospheric variations, and which, in the first warmth of spring, send 
up their accumulated juices with force through the uppermost parts of the 
tree.— Field Naturalist. 
