Every student of nature must be greatly struck with the analogies between different portions of the visible creation. These spiral structures which we have just examined are almost identical in appearance, and to some extent in their function, with the threads that are coiled within the breathing tubes of insects. This is in both cases twofold, namely, to give support and elasticity to a delicate membrane, and to preserve the tube in its proper form, despite the bending to which it may be subjected. When we come to the anatomy of the insect in a future page we shall see this structure further exemplified. In some cases the deposit, instead of forming a spiral coil, is arranged in a series of rings, and the vessel is then termed “annulated.” A very good example of this formation is given in Fig. 6, which is a sketch of such a vessel, taken from a stalk of the common rhubarb. To see these ringed vessels properly, the simplest plan is to boil the rhubarb until it is quite soft, then to break down the pulpy mass until it is flattened, to take some of the most promising portions with the forceps, lay them on the slide and press them down with a thin glass cover. They will not be found scattered at random through the fibres, which elsewhere present only a congeries of elongated cells, but are seen grouped together in bundles, and with a little trouble may be well isolated, and the pulpy mass worked away so as to show them in their full beauty. As may be seen in the illustration, the number of the rings and their arrangement is extremely variable. A better, but somewhat more troublesome, plan is to cut longitudinal sections of the stem, as described in our concluding chapter, when not only the various forms of cells and vessels, but their relations to each other, will be well shown. The numerous crystals of oxalate of lime, which make rhubarb so injurious a food for certain persons, will also be well seen. These crystals are called “raphides,” and are to be found in very many plants in different forms. II.