MALADIE ET SYMBIOSE 259 



of these glands, inter alia to digest the captured insects. I would, 

 therefore, rather explain the habitat by the appetites than vice 

 versa. The sun-dew family, in my opinion, suffers from an 

 in-feeding diathesis. Nor is it a " physical " explanation, as some 

 imagine, to say that the sun-dew's reproductive powers are 

 increased because they are sensitive to degenerative influences, 

 when all that has happened is this : the symbiotic restraint has 

 gone and with it the restraint of propagation. This, however, 

 so far from being a genuine increase of reproductive power, is 

 only equivalent to a dissociation of such power and produces 

 weakness. 



As the matter is one of some importance, I feel justified 

 in quoting Prof. Henslow further, and at some length : 



The root of the land plant is solid with a central and circumferential 

 mass of cellular tissue, together with clusters of wood-fibres arranged in 

 a circular manner. In the aquatic root large holes occur in the former 

 tissues, and the wood is greatly reduced in quantity, as the water supports 

 the plant. As roots must be well aerated for respiration some trees growing 

 in swamps have their roots with ascending parts like knees or poles 

 rising out of the ground, which are more or less hollow and filled with air. 

 In herbs, the pith of stems is like a sponge, only the holes are filled with air, 

 as occurs in those of rushes ; so, too, is the surface of the root of the marsh 

 samphire, so abundant on salt-marshes. These are compensating structures 

 to overcome the injurious effects of too much and insufficiently aerated water. 



Thick stems, such as the rhizomes of water-lilies, and the aerial stems 

 of palms, as well as all other monocotyledons, have very degenerate 

 characters, i.e. if we regard an oak tree, for example, as the type of what 

 a stem should be. Timber trees put on annual cylinders of wood, thereby 

 making the stem strong enough to support their own weight and that of 

 the mass of foliage. 



If life in the water is thus in many ways easier than upon 

 the land the plant being suffered to exist epiphytically as it 

 were upon the supporting water it is yet not without grave 

 disabilities and dire handicaps. . It is better for a plant to live 

 on the land and to support itself by vigorously drawing on soil 

 and atmosphere. There is almost a sociological turn in Prof. 

 Henslow's passage when he refers to a " type of what a stem 

 should be." This desideratum is fulfilled where we have a plant 

 duly drawing on the mineral substances of the soil and thus 

 evolving a complete vascular system a matter of the utmost 

 importance to organic life generally, which should be consistently 

 taken into account. 



" In aquatic stems," Prof. Henslow goes on to say, " the 



