YOUNGdo not contain a solid core of wood. Most of their stiffness is due to a characteristic device which is responsible for the firmness of all young parts of plants. It is common knowledge that the hardness of a pneumatic tyre depends on the balance between the tendency of the inflated tube to expand and the restraint imposed by the outer cover. In young parts of plants we find a similar but more complicated arrangement. The plant is covered by a thin and relatively strong skin which is not very elastic and resists stretching. Inside this there is a mass of soft material composed of many cells pressing against one another and against the outer skin, since they are distended with water. The balance between the outward pressure set up by these cells and the resistance to stretching offered by the epidermis (that is the outer skin of the plant) gives rigidity to the whole system.
It is easy to demonstrate that there are strains in young plant material. For example, if the cylindrical floweringof the dandelion is slit into several strips, each strip curls with the skin on the inner and shorter side of the curl. So long as the stem is intact, the skin is stretched and the soft cells within are held under restraint. Cutting releases the tensions, the skin is now free to contract and the soft cells to expand, and curling necessarily follows.
The young and softof most land plants contain some strands of particularly strong material; these assist in stiffening the stem and they have an important role in preventing
sudden and localised bending. In general, these strands, which run lengthwise, lie close inside the skin, giving a condition comparable with the arrangement of material in a hollow metal column. This is just the arrangement which offers the most effective resistance to buckling; in the hollow stems of grasses and many other plants, the efficiency of the arrangement is increased by the partitions which cross the stem here and there. In some plants the centre of the stem is occupied by a pith which, from a mechanical point of view, acts in relation to bending and buckling in the same way that a filling of sand acts in a thin metal tube that is being made into a coil.
The strands of strong material close to the outside of the stem are very apparent in the long flowering stems of the grasses, which are wonderful examples of mechanical efficiency, for the contrast between diameter and length is remarkable. In other stems, such as the square stems of mint, there is a strand of strong material in each corner; the whole stem may be compared with an erect X-girder which resists excessive bending very well. When it is remembered that one of the main purposes of an erect stem is to hold theand in a good light, the importance of structural devices which check overthrow is at once apparent.
The materials which contribute to the mechanical efficiency of the stem are often of great strength. Linen is made from the strands of the stems of flax, and cordage from similar material from hemp. Some vegetable strands have a breaking strain which is one quarter that of a steel wire of equal diameter. They stretch freely, but their breaking-point lies close to the extreme point to which they can be stretched. Consequently there is little chance that the plant will be permanently stretched sufficiently to cause deformation; this would be more injurious than breaking, for it would ruin the adjustment of the internalof the stem, cause the death of some of the parts, and provide a starting-point for disease.
Owing to their elasticity, the young parts of plants move freely in the wind, and, unless the wind is strong enough to cause a break, the plant recovers its form when the wind ceases to bend it. The mechanical strands often lie in the stem close to the delicate strands of minute tubes which are responsible for the transport of liquids about the plant, and they protect them against sudden buckling and injury.