In these cases, however, the tertiary aggregate, so far as it is formed, is but very feebly integrated; and its integration is but temporary. For not only do these younger fronds that bud out from the mid-ribs of older fronds, develop rootlets of their own; but as soon as they are well grown and adequately rooted, they dissolve their connexions with the parent-fronds, and become quite independent. From these transitional forms we pass, in the higher _Jungermanniaceæ_, to forms composed of many fronds that are permanently united by a continuous stem. A more-developed aggregate of the third order is thus produced. But though, along with increased definiteness in the secondary aggregates, there is here an integration of them so extensive and so regular, that they are visibly subordinated to the whole they form; yet the subordination is really very incomplete. In some instances, as in _Radula complanata_, Fig. 47, the leaflets develop roots from their under surfaces, just as the primitive frond does; and in the majority of the group, as in _J. capitata_, Fig. 48, roots are given off all along the connecting stem, at the spots where the leaflets or frondlets join it: the result being that though the connected frondlets form a physical whole, they do not form, in any decided manner, a physiological whole; since successive portions of the united series, carry on their functions independently of the rest. Finally, the most developed members of the group, whether lineally descended from the less developed or from an early type common to the two, present us with tertiary aggregates which are physiologically as well as physically integrated.[5] Not lying prone like the kinds thus far described, but growing erect, the stem and attached leaflets become dependent upon a single root or group of roots; and being so prevented from carrying on their functions separately, are made members of a compound individual: there arises a definitely-established aggregate of the third degree of composition. The facts as arranged in the above order are suggestive. Minute aggregates, or cells, the grouping of which we traced in § 182, showed us analogous phases of indefinite union, which appeared to lead the way towards definite union. We see here among compound aggregates, as we saw there among simple aggregates, the establishment of a specific form, and a size that falls within moderate limits of variation. This passage from less definite extension to more definite extension, seems in the one case, as the other, to be accompanied by the result, that growth exceeding a certain rate, ends in the formation of a new aggregate, rather than an enlargement of the old. And on the higher stage, as on the lower, this process, irregularly carried out in the simpler types, produces in them unions that are but temporary; while in the more-developed types, it proceeds in a systematic way, and ends in the production of a permanent aggregate that is doubly compound.