288 
THE PLANT WORLD. 
surrounding water. The plants are terrestrial forms adapted to 
an aquatic habit rather than descendants of plants in which the 
functions of absorption and excretion are not localized. These 
rooted aquatics are important contributors to the plankton food 
supply, because when living they organize matter that may be 
used as food and in death they yield important salts and organic 
substances to the water. 
In the stocking of ponds for fish culture care should be taken 
to have a good soil for the bottom; not a stiff clay nor sand, but 
a good loamy soil, such as is favorable for land plants. The 
species allowed to grow should be those which are known to 
possess roots and to be very dependent upon the soil. 
The Cause of Diatom Motion. —No physiological phenome¬ 
non of cryptogamic plants has been more mysterious, nor given 
rise to more theories than the motion of diatoms. A new theory 
has recently been advanced by Jackson.* The motion is not 
due to cilia, flagella, protoplasmic processes, rhythmical motion 
of surface film, nor to the endosmotic and exosmotic currents, as 
other students have suggested. 
The “ true nature of the motion ” was suggested to the writer 
of the article referred to, by observing the motion of a lithia 
tablet dissolving in a glass of water. This motion is due to the 
evolution of bubbles of carbon-dioxid, and when tablets were 
made of the shape of diatoms their motion was imitated exactly. 
Imitation diatoms were then made of aluminium, similar to the 
plants in shape and surface markings. “ When placed in strong 
caustic soda solution the movements of the metal produced by 
the evolution of hydrogen gas again duplicated those of the dia¬ 
tom in a remarkable manner.” 
In the diatom itself the evolution of gas is caused by the photo¬ 
synthetic activity of the chlorophyll bands in the presence of light. 
The writer concludes that “ the motion of diatoms is caused by 
the impelling force of the bubbles of oxygen evolved, and that 
the direction of movement is due to the relatively larger amount 
of oxygen set free first from the forward and then from the rear 
half of the organism.” C. S. G. 
* Jackson, D. D. Movements of Diatoms and other Microscopic Plants. 
Amer. Nat. 39: 28 7. 1905. 
