Even in the case of the roots, which appear to be very irregular in their ramification, it has been found that, in the first instance at least, the rootlets or fibrils are arranged in regular order one over another, in a certain determinate number of vertical ranks, generally either in two or in four, sometimes in three or in five series. This regularity of arrangement (Rhizotaxy), first carefully studied by M. Clos, is connected with the disposition of the fibro-vascular bundles in the body of the root. This primitive regularity is soon lost as the plant grows. In the case of the leaves there are two principal modes of arrangement, dependent, as it would seem, on their simultaneous or on their successive development; thus, if two leaves on opposite sides of the stem are developed at the same time, we have the arrangement called opposite; if there are more than two, the disposition is then called verticillate or whorled. On the other hand, if the leaves are developed in succession, one after the other, they are found to emerge from the stem in a spiral direction. In either case the leaves are arranged in a certain regular manner, according to what are called the laws of Phyllotaxis, which need not be entered into fully here; but in order the better to estimate the teratological changes which take place, it may be well to allude to the following circumstances relating to the alternation of parts. The effect of this alternation is such, that no two adjacent leaves stand directly over or in front one of the other, but a little to one side or a little higher up. Now, in the alternate arrangement the successive leaves of each spiral cycle alternate one with another till the coil is completed. For the sake of clearness this may be illustrated thus:--Suppose the spiral cycle to comprise five leaves, numbered 1, 2, 3, 4, 5, then 2 would intervene between 1 and 3, and so on, while the sixth leaf would be the commencement of a new series, and would be placed exactly over 1. This arrangement may be thus formularised: 6 7 8 9 10 1 2 3 4 5 In the verticillate or simultaneous arrangement of leaves the case is somewhat different. Let us suppose a whorl of eight leaves, surmounted by a similar whorl of eight. In such a case it will generally be found that the whorls alternate one with another, as may be represented by this symbol: 9 10 11 12 13 14 15 16 1 2 3 4 5 6 7 8 The simplest illustration of this arrangement is seen in the case of decussate leaves, where those organs are placed in pairs, and the pairs cross one another at right angles. This may be expressed by the following symbol: 7 8 5 6 3 4 1 2