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.
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.
Where, as in types like these, the morphological units show an inherent tendency to arrange themselves in a manner which is so far constant as to give characteristic proportions, we may say that there is a recognizable compound individuality. Considering the Thallophytes which grow in this way apart from their kinships, and wholly with reference to their morphological composition, we might not inaptly describe them as pseudo-foliar.
§ 184. Another mode in which aggregation is so carried on as to produce a compound individuality of considerable definiteness, is variously displayed among other families of Algæ. When the cells, instead of multiplying longitudinally alone, and instead of all multiplying laterally as well as longitudinally, multiply laterally only at particular places, they produce branched structures.
Indications of this mode of aggregation occur among the Confervoideæ, as shown in Figs. [22, 23]. Though, in some of the more-developed Algæ which exhibit the ramified arrangement in a higher degree, the component cells are, like those of the lower Algæ, united together end to end, in such way as but little to obscure their separate forms, as in Cladophora Hutchinsiæ, Fig. [31]; they nevertheless evince greater subordination to the whole of which they are parts, by arranging themselves more methodically. Still further pronounced becomes the compound individuality when, while the component cells of the branches unite completely into jointed cylinders, the component cells of the stem form an axis distinguished by its relative thickness and complexity. Such types of structures are indicated by Figs. [32, 33]—figures representing small portions of plants which are quite tree-like in their entire outlines. On examining Figs. [34, 35, 36], which show the structures of the stems in these types, it will be seen, too, that the component cells in becoming more coherent, have undergone changes of form which obscure their individualities more than before. Not only are they much elongated, but they are so compressed as to be prismatic rather than cylindrical. This structure, besides displaying integration of the morphological units carried on in two directions instead of one; and besides displaying this higher integration in the greater merging of the individualities of the morphological units in the general individuality; also displays it in the more pronounced subordination of the branches and branchlets to the main stem. This differentiation and consolidation of the stem, brings all the secondary growths into more marked dependence; and so renders the individuality of the aggregate more decided.
