CIRCULATION AND FORMATION OF WOOD IN PLANTS. 575 



tive thickenings of parts which distinguish the individual from others 

 of its kind. And then, in certain irregularly growing plants, such as 

 Cactuses and Euphorbias, where the strains fall on parts that do 

 not correspond in successive individuals, we distinctly trace a direct 

 relation between the degrees of strain and the rates of these changes 

 which result in dense tissue. I will not occupy space in detailing 

 the evidence of this relation, which is conspicuous in the orders 

 named, butwill pass to thequestion — What arethephysicalprocesses 

 by which intermittent mechanical strains produce this deposit of 

 resistant substance at places where it is needed to meet the strains? 

 We have not to seek far for an answer. If a trunk, a bough, a 

 shoot, or a petiole, is bent by a gust of wind, the substance of its 

 convex side is subject to longitudinal tension: the substance of its 

 concave side being at the same time compressed. This is the pri- 

 mary mechanical effect. There is, however, a secondary mechani- 

 cal effect, which here chiefly concerns us. That bend by which the 

 tissues of the convex side are stretched, also produces lateral com- 

 pression of them. Buttoning on a tight glove and then closing the 

 hand, will make this necessity clear : the leather, while it is strained 

 along the backs of the fingers, presses with considerable force on the 

 knuckles. It is demonstrable that the tensions of the outer layer 

 of a mass made convex by bending, must, by composition of forces, 

 produce at every point a resultant at right angles to the layer be- 

 neath it ; that, similarly, the joint tensions of these two layers must 

 throw a pressure on the next deeper layer ; and so on. Hence, if 

 at some little distance beneath the surface of a stem, twig, or leaf- 

 stalk, there exist longitudinal tubes, these tubes must be squeezed 

 each time the side of the branch they are placed on becomes convex. 

 Modifying the illustration just drawn from the clenched hand will 

 make this clear. When, on forcibly grasping something, the skin is 

 drawn tightly over the back of the hand, the whitening of the 

 knuckles shows how the blood is expelled from the vessels below 

 the surface by the pressure of the tightened skin. If, then, the sap- 

 vessels must be thus compressed, what will happen to the liquid they 

 contain ? It will move away along the lines of least resistance. 

 Part, and probably the greater part, will escape lengthways from the 

 place of greatest pressure : some of it being expelled downwards, 

 and some of it upwards. But, at the same time, part of it will be 

 likely to ooze through the walls of the tubes. If these walls are so 

 perfect as to permit the passage of liquid only by osmose, it may 

 still be inferred that the osmose will increase under pressure ; and 

 probably, under recurrent pressure, the places at which the osmotic 

 current passes most readily will become more and more permeable, 

 until they eventually form pores. At any rate it is manifest that 

 where pores and slits exist, whether thus formed or formed in any 

 other way, the escape of sap into the adjacent tissue at each bend 



