98 ALSO THE RAPIDITY OF THE CIRCULATION^. 



genial sunshine of the early spring stimulates it to increased activity. 

 The general increased temperature of the air does not produce this ac- 

 celeration in so remarkable a manner as the direct rays of the sun. The 

 sap will flow and circulate on the side of a tree on which the sunshine 

 falls, while it remains sensibly stagnant on the other. This is shown by 

 the cutting down similar trees at more and more advanced periods of 

 the spring, and immersing their lower extremities in coloured solutions. 

 The wood and bark on one side of the tree will be coloured, while, on 

 the other, both will remain unstained. If a similar difference in the 

 comparative rapidity of the circulation on opposite sides of a trunk or 

 branch be supposed to prevail more or less throughout the year, we can 

 readily account for the annual layers of wood being often thicker on 

 the one half of the circumference of the stem than on the other. 



The sap is generally supposed to flow most rapidly during the spring, 

 but if trees be cut dow^ at different seasons, and immersed as above 

 described, the coloured solution, according to Boucherie, reaches the 

 leaves most rapidly in the autumn.* 



The heat of the day, other circumstances being the same, materially 

 affects, for the time, the rapidity of the circulation. The more rapidly 

 watery and other vapours are exhaled from the leaves, the more quick- 

 ly must the sap flow upwards to supply the waste. If on two succes- 

 sive days the loss by the leaves be, as in the experiment of Hales, above 

 described, (p. 90,) as 2 to 3, the ascent of the sap must be accelerated 

 or retarded in a similar proportion. Hence, every sensible variation in 

 the temperature and moisture of the air, must also, to a certain extent, 

 modify the flow of the sap ; must cause a greater or less transport of that 

 food which the earth supplies, to be carried to every part of the plant, 

 and must thus sensibly affect the luxuriance and growth of the whole. 



But the persistance of the leaves is a generic character, which has 

 considerable influence upon the circulation in the evergreens. In the 

 pine and the holly, from which the leaves do not fall in the autumn, the 

 sap ascends and descends during all the colder months, — at a slower 

 rate, it is true, than in the hot days of summer, yet much more sensibly 

 than in the oak and ash, which spread their naked arms through the 

 wintery air. This is illustrated by the experiments of Boucherie, who 

 has observed that in December and January the entire wood of resinous 

 trees may be readily and thoroughly penetrated by the spontaneous as- 

 cent of saline and other solutions, into which their stems may be im- 

 mersed. 



III. From what has just been stated, it will appear that the mechani- 

 cal functions of the stem are subject to precisely the same influences as 

 the ascent of the sap. As the tree advances in age, the vessels of the 

 interior will become more or less obliterated, and the general course of 

 the sap will be gradually transferred to annual layers, more and more 



* Boucherie makes a distinction, not hitherto insisted upon by physiologists, between the 

 circulation on the surface of"the tree by wflich the buds and youns; twigs are supported, and 

 the interior circulation, which is not perfect until a latter period of the year. Hence in the 

 spring, though the sap is flowing rapidly through the bark and tiie newest wood, coloured 

 solutions will not penetrate the interior of the tree with any degree of rapidity. In autumn, 

 on the other hand — when the fear of approaching winter has already descended upon the 

 bark — the time of most active circulation has only arrived for the interior layers of the older 

 wood. It is this season consequently that he finds most favourable for impregnating the 

 trunks of trees with those soluiions which are likely to preserve them from decay.— il^in. de 

 CJiim. et de Phys.^ Ixxiv., p. 135. 



