274 PHYSIOLOGICAL DEVELOPMENT. 



besides the paucity of vessels in ordinary water-plants, thei 

 are cases of much more marked divergence from this typical 

 internal structure. These exceptional cases occur undei 

 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 

 Balanophorce and Rafflesiacece are recognizable as Phsenogams 

 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 

 Phamogams 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 mis- 

 take 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. The 



other cases occur where circulation is arrested or checked in 

 a different way ; namely, in plants that are wholly submerged. 

 These are the Podostemaceoe. Clothing as they do the sub- 

 merged rocks, their roots play the part of rhizomes, being- 

 attached to the substratum by hairs and other processes, and 

 having the leaf-bearing and flower-bearing shoots on their 

 surfaces. The latter spread out more or less horizontally and 

 are also fixed to the substratum in the same manner as the 

 roots. Observe then the connexion of facts. One of these 

 Podostemaceoe needs no internal stiffening substance, for it 

 exists in a medium of its own specific gravity; and being in 

 a position to absorb water over its entire surface, it has no 

 need for a circulation of crude sap — nor, indeed, in the 

 absence of evaporation from any part of its surface, could 



