ROOTS or CERTAIN PALMS. 283 



Warming has described a system of intercellular spaces in the aerial roots of Rhizo- 

 phora Mangle, Linn., to which he ascribes the function of helping to float the young 

 plants ; and shows that these spaces are kept distended by the presence in them of 

 branched cells [trichoblasts), and that in this way collapse due to the heat of the sun 

 may be avoided. 



G. Karsten makes further mention of such intercellular spaces in the Rhizophoracea?, 

 and recognizing their function calls them " pnemnatophores." In this he coins a word 

 having analogy with the term " pneumatode," which Jost has applied to all openings of 

 the nature of lenticels or stomata. G. Karsten mentions Palms also in which pneuma- 

 tophores occur. In the case of the Palm-roots in question, there are no trichoblasts, and 

 the duty of keeping the spaces open can scarcely be ascribed to the sclerenchy ma-strands. 

 On the other hand, it must be noted that the spaces are very much larger than those of 

 the E/hizophoraceae ; and, further, the peripheral sclerotic ring previously mentioned 

 guards against collapse. Similarly, as G. Karsten has shown, a woody ring aids in 

 preventing obliteration by compression in the case of the Rhizophoracese. It may be 

 that the development of sclerenchymatous strands or trichoblasts leads up to the forma- 

 tion of the intercellular spaces associated with them. 



There remains to be pointed out the surprising increase of facility for movement of a 

 body of air due to even a slight increase in diameter of a pneumatophore. Fluids, 

 whether liquids or gases, have, owing to their viscosity, a different mode of flow in wide 

 and in narrow channels. The speed with which a body of fluid can be transferred through 

 a capillary tube increases as the fourth power of tlie tube's diameter. As it is evident 

 from the work of Clerk Maxwell and others that this is true for gases as well as liquids, 

 the advantage of increase of diameter of the pneumatophores is obvious. 



In the Palm-roots examined nothing of the nature of pneumatodes was discovered. 

 This finds a parallel in G. Karsten's observation that while other Hhizophoraceoe have 

 pneumatodes in the roots, especially at places of branching, species of Ceriops have none 

 in the root, but have them on the stem close to the ground. 



It is not likely that, in the case of such roots springing from a monocotyledonous stem, 

 the stem-system of pneumatophores could be separate from the root-system ; but as the 

 material for the present investigation did not include stem, it was not possible to test 

 the continuity. 



The foresxoino: considerations lead to the followinsr conclusions : — 



Aerial roots from several species of Palms are polystelic in their older thicker parts, 

 and there is a continuous transition to normal monostelic structure in their younger 

 thinner parts. 



This difference in structure is due to a continuous change in the mode of differen- 

 tiation of the apical meristem. 



The whole structure of the older aerial parts is such as to fit them for withstanding 

 both pressure as props or tension as stays ; while the thinner subterranean parts are 

 normal in conformity with the normality of their functions and environment. 



In correlation with the bulk of these roots, and the absence from them of pneumatodes, 

 there is a conspicuous formation of pneumatophores. 



SECOND SERIES. — BOTANY, VOL. V. 2 S 



