A natural concomitant of the mode of growth here described, is that the stem, while it increases longitudinally, increases scarcely at all transversely: hence the old name Acrogens. Clearly the transverse development of a stem is the correlative, partly of its function as a channel of circulation, and partly of its function as a mechanical support. That an axis may lift its attached leaves into the air, implies thickness and solidity proportionate to the mass of such leaves; and an increase of its sap-vessels, also proportionate to the mass of such leaves, is necessitated when the roots are all at one end and the leaves at the other. But in the generality of Acrogens, these conditions, under which arises the necessity for transverse growth of the axis, are absent wholly or in great part. The stem habitually creeps below the surface, or lies prone upon the surface; and where it grows in a vertical or inclined direction, does this by attaching itself to a vertical or inclined object. Moreover, throwing out rootlets, as it mostly does, at intervals throughout its length, it is not called upon in any considerable degree, to transfer nutritive materials from one of its ends to the other. Hence this peculiarity which gives their name to the Acrogens, now called Archegoniates, is a natural accompaniment of the low degree of specialization reached in them. And that it is an incidental and not a necessary peculiarity, is demonstrated by two converse facts. On the one hand, in those higher Acrogens which, like the tree-ferns, lift large masses of foliage into the air, there is just as decided a transverse expansion of the axis as in dicotyledonous trees. On the other hand, in those Dicotyledons which, like the common Dodder, gain support and nutriment from the surfaces over which they creep, there is no more lateral expansion of the axis than is habitual among Acrogens or Archegoniates. Concluding, as we are thus fully justified in doing, that the lateral expansion accompanying longitudinal extension, which is a general characteristic of Phanerogams as distinguished from Archegoniates, is nothing more than a concomitant of their usually-vertical growth;[10] let us now go on to consider how vertical growth originates, and what are the structural changes it involves.