§ 284. The inferior _Algæ_, along with little unlikeness of parts, show us little mutual dependence of parts. Having surfaces similarly circumstanced everywhere, much physiological division of labour cannot arise; and therefore there cannot be much physiological unity. Among the superior _Algæ_, however, the differentiation between the attached part and the free part is accompanied by some integration. There is evidently a certain transfer of materials, which is doubtless facilitated by the elongated forms of the cells in the stem, and probably leads to the formation of dense tissue at the places of greatest strain, in a way akin to that recently explained in other cases. And where there is this co-ordination of actions, the parts are so far mutually dependent that each dies if detached from the other. That though the organization is so low neither part can reproduce the other and survive by so doing, is probably due to the circumstance that neither part contains any considerable stock of untransformed protoplasm, out of which new tissues may be produced. Fungi and Lichens present no very significant advances of integration. We will therefore pass at once to the Archegoniates. In those of them which, either as single fronds or strings of fronds, spread over surfaces, and which, rooting themselves as they spread, do not need that each part should receive aid from remote parts, there is no developed vascular system serving to facilitate transfer of nutriment: the parts being little differentiated there is but little integration. But along with assumption of the upright attitude and the accompanying specializations, producing vessels for distributing sap and hard tissue for giving mechanical support, there arises a decided physiological division of labour; rendering the aërial part dependent on the embedded part and the embedded part dependent on the aërial part. Here, indeed, as elsewhere, these concomitant changes are but two aspects of the same change. Always the gain of power to discharge a special function involves a loss of power to perform other functions; and always, therefore, increased mutual dependence constituting physiological integration, must keep pace with that increased fitting of particular parts to particular duties which constitutes physiological differentiation.