§ 183. An approach towards plants which unite these characters, may be traced in such forms as _Bangia ciliaris_, Fig. 24. The multiplication of cells here takes place, not in a longitudinal direction only, but also in a transverse direction; and the transverse multiplication being greater towards the middle of the frond, there results a difference between the middle and the two extremities--a character which, in a feeble way, unites all the parts into a whole. Even this slight individuation is, however, very indefinitely marked; since, as shown by the figures, the lateral multiplication of cells does not go on in a precise manner. [Illustration: Fig. 24.] From some such type as this there appear to arise, through slight differences in the modes of growth, two closely-allied groups of plants, having individualities somewhat more pronounced. If, while the cells multiply longitudinally, their lateral multiplication goes on in one direction only, there results a flat surface, as in the genus _Ulva_ (Sea-lettuce) or in the upper part of the thallus of _Enteromorpha Linza_, Fig. 25; or where the lateral multiplication is less uniform in its rate, in types like Fig. 26. But where the lateral multiplication occurs in two directions transverse to one another, a hollow frond may be produced--sometimes irregularly spheroidal, and sometimes irregularly tubular; as in _Enteromorpha intestinalis_, Fig. 27. And often, as in _Enteromorpha compressa_, Fig. 28, and other species, this tubular frond becomes branched. Figs. 29 and 30 are magnified portions of such fronds, showing the simple cellular aggregation which allies them with the preceding forms. [Illustration: Figs. 25–30.] In the common _Fuci_ of our coasts, other and somewhat higher stages of this integration are displayed. We have fronds preserving something like constant breadths and dividing dichotomously with approximate regularity. Though the subdivisions so produced are not to be regarded as separate fronds, but only as extensions of one frond, they foreshadow a higher degree of composition; and by the comparatively methodic way in which they are united, give to the aggregate a more definite, as well as a more complex, individuality. Many of the higher lichens exhibit an analogous advance. While in the lowest lichens, the different parts of the thallus are held together only by being all attached to the supporting surface, in the higher lichens the thallus is so far integrated that it can support itself by attachment to such surface at one point only. And then, in still more developed kinds, we find the thallus assuming a dichotomously-branched form, and so gaining a more specific character as well as greater size.