We have seen several examples of ringed tissues tending to the spiral form, and it is but natural that we should expect to find spiral tissues tending to the ring.
In the accompanying illustration the two left-hand figures represent the curiously modified ringed tissue which is to be found in the sugar-cane, the left-hand figure being much more magnified than the other.
The other figures represent four examples of vegetable spiral tissues, in which it will be seen that there is a tendency to form rings, and that if a number of rings were substituted for the spiral, and the object viewed in a slanting direction, it would be almost impossible to distinguish between the ring and the spiral.
Among the most remarkable of these examples are the two right-hand figures. That on the extreme right represents a spiral vessel taken from the so-called root, or “rhizome,” of the Water-lily, and the other is a similar vessel taken from a branch of the Yew-tree. It has been suggested that to this spiral structure is due the proverbial elasticity of the yew-tree, which has from time immemorial rendered it the best wood for the manufacture of bows.
Diving and Divers.
It has already been mentioned that the flexible tubes used by modern divers are constructed on the model of several structures belonging to the animal and vegetable kingdoms.
We will now see how they are utilised.
In the earlier stages of the diver’s art the Diving-bell afforded the only means of gaining access to the bed of the sea, even in comparatively shallow waters. The mode in which this result was obtained was simple enough, and though it carried with it the germs of still greater improvements, was but limited and uncertain in its action.
The reader is probably aware that if a vessel be filled with air, no liquid can obtain admittance until a corresponding amount of air be set free. Suppose, for example, that an empty tumbler be inserted over a basin of very clean water, and pressed downwards, it will be found that scarcely any water will enter it, the air having taken up all the available space, and only allowing as much space as may be accounted for by its faculty of compression.