—— 
La 
stop. The cells of the cortex then enlarge; at first they keep their form, 
but afterwards they become spherical and finally put out processes each 
of which keeps in contact with a corresponding one from an adjacent 
cell. ‘This enlargement of the cortex ruptures the epidermis and the 
growth of the inner layers separates the outer ones, so that the epidermis 
and outer layers flare back from both ends of the swollen zone; its surface 
is then mealy in appearance and white because of the contained air. As 
the pneumathode ages, the cap and all the outer tissues beyond the open 
zone slough off; the strongly lignified stele gives it stability and its sharp 
point will protect it against mechanical injury, if protection is needed. 
The cells of the open tissue necessarily promptly die, but their walls 
remain firm, their surfaces become granular, and in this condition they 
can not be wetted, so that the large amount of air contained between 
them can not be displaced. The cells next to the stele, and those at the 
base of the pneumathode—that is, those toward the parent root—enlarge 
moderately and become spherical, and thus form intercellular spaces of 
some size; their surfaces also become granular and their walls very thick, 
thus insuring the permanency of open aérial communication through the 
pneumathode to the tissue of the parent root, which has the most abundant 
system of intercellular spaces—that is, the cortical parenchyma. 
Roots which have suffered metamorphosis to serve as pneumathodes 
have been encountered in many plants, and have been most thoroughly 
studied in this part of the world,* but in all previously known cases they 
are formed as a response to the wetness of the environment. In many 
plants which grow in wet places, either frequently or invariably, pneuma- 
thodes have become normal structures; in many others, whose roots only 
exceptionally find themselves where the supply of air is cut off by water, 
pneumathode-like structures form as abnormalities.*| In plants whose 
habitat is such that pneumathodes have become a normal structure, the 
roots which serve this purpose have usually aequired a negative geotro- 
pism, adapting themselves to the direction in which the air is to be found. 
This is true of Phoenix, whose pneumathodes, as figured by Tischler,® are 
very similar to those of Cocos, 
In distinction to all other known pneumathodes, those of Cocos are 
demanded by the structure of the plant without regard to what its envi- 
ronment may be. ‘They form on roots in water, in firm ground, in loose 
sand, and in the air. In soil containing free air, where the roots normally 
grow and the formation of the pneumathodes is under the control of 
* Karsten: Ueber die Mangrove-Vegetation im malayischen Archipel. Biblio- 
theca botanica (1891), Heft 22. 
*The same is true of other parts of the plant as well. See Sorauer: “Ueber 
Intumescenzen.” Ber, bot. Gesell. (1891), 17, 456, and my note on Haberlandt’s 
new organ on Conocephalus, Bot. Gaz. (1902), 33, 300. 
* Tischler, G.: Ueber das Vorkommen von Statolithen bei wenig oder gar nicht 
geotropischen Wurzeln. Flora (1905), 94, 35. 
