§ 278. A terrestrial plant that grows vertically needs no marked modification of its internal tissues, so long as the height it reaches is very small. As we before saw, the spiral or cylindrical rolling up of a simple cellular frond, or the more bulky growth of a simple cellular axis, may give the requisite strength; and the requisite circulation may be carried on through the unchanged cellular tissue. But in proportion as the height to be attained and the mass to be supported increase, the supporting part must acquire greater bulk or greater density, or both; and some modification that shall facilitate the transfer of nutritive liquids must take place. Hence, in the inner tissues of plants we may expect to find that structural changes answering to these requirements become marked, as the growth of the aërial part becomes great. Facts correspond with these expectations. Among the humbler Cormophytes, which creep over or raise themselves but little above, the surfaces they flourish upon, there is scarcely any internal differentiation: the vascular and woody structures, if not in all cases absolutely unrepresented, are rarely and very feebly indicated. But among the higher types--the Ferns and Lycopodiums--which raise their fronds to considerable heights, there are vascular bundles and hard tissues like wood; and by the Tree-Ferns massive axes are developed. That the relation which thus shows itself among Cryptogams is habitual among Phænogams, scarcely needs saying. Phænogams, however, are not universally thus characterized in a decided way. Besides the comparative want of woody tissue in flowering plants of humble growth, and besides the paucity of vessels in ordinary water-plants, there are cases of much more marked divergence from this typical internal structure. These exceptional cases occur under exceptional conditions, and are highly instructive. They are of two kinds. One group of them is furnished by certain plants which are parasitic on the exposed roots of trees--parasitic not partially, as the Mistletoe, but to the extent of subsisting wholly on the sap they absorb. Fungus-like in colour and texture, and having scales for leaves, these _Balanophoræ_ and _Rafflesiaceæ_ are recognizable as Phænogams by scarcely any other traits than their fructifications. Along with their aborted leaves and absence of chlorophyll, there is a great degradation of those internal tissues by which Phænogams are commonly distinguished. Though Dr. [now Sir J.] Hooker has shown that they are not, as some botanists thought, devoid of spiral vessels; yet, as shown by the mistake previously made in classifying them, their appliances for circulation are rudimentary. And this trait goes along with a greatly-simplified distribution of nutriment. In the absence of leaves there can be but little down-current of sap, such as leaves usually supply to roots: there cannot be much beyond an upward current of the absorbed juices.