THE INNER TISSUES OF PLANTS. 257 



besides tlie 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 that 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 

 BalanopiiorcB and Rafflesiacece are recognizable as Phoenogams 

 by scarcely any other traits than their fructifications, Along 

 with their abortive leaves and absence of chlorophyll, there 

 is a great degradation of those internal tissues by which 

 Phaenogams are commonly distinguished. Though Dr. 

 Hooker has shown that they are not, a3 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 

 nutriment, such as leaves usually supply to roots : there 

 cannot be much beyond an upward current of the absorbed 

 juices. The other cases occur where circulation 



is arrested or checked in a different way ; namely, in 

 plants that are wholly submerged. These are the Podo- 

 stemones, which are aquatic even to the extent of flowering 

 under water. Clothing as they do the submerged rocks 

 in tropical rivers, their roots, like those of the Algce, serve 

 only for attachment ; their foliar expansions, frond-like in 

 shape, are everywhere bathed by the water; and their organs 

 of fructification never exposed to the air, but perhaps aided 

 in their functions by water-insects instead of air-insects, are 

 the only marked signs of kinship to other Phaenogams. Observe 

 then the connexion of facts. One of these Poclcstemones needs 

 no internal stiffening substance, for it exists in a medium of 

 its own specific gravity ; and having no unlikeness between 



