Structure and Origin of “ Cladophora Balls." 9 
presence of specialised filaments which firmly bind together the 
vegetative branches. Growth is slow and takes place in a tan¬ 
gential as well as a radial direction. This tangential growth and 
probably also the successive destruction of the older branches, 
causes a cavity to form in the centre of the ball. Usually this 
cavity is filled with water, but during the months of May and April 
they enclose a considerable quantity of gas which enables them to 
float on the surface while they remain at the bottom during the 
remainder of the year. 1 
The presence of the gas in the interior of the ball is explained 
by the fact that as the intensity of the light is increased the cells 
on the inner part of the ball take part in assimilation as well as 
those in the exterior, and the carbon-dioxide from these escapes into 
the interior of the ball. The close texture of the peripheral part 
prevents the rapid escape of the gas and in time sufficient is 
collected to float the ball. The majority of these floating balls 
probably perish owing to the strong illumination which is inimical 
to the growth of the plant. 
The regular periodic appearance of the balls at the surface 
during April and May is explained as follows. In normal conditions 
the zone occupied by the balls would be too intensely lighted for 
the growth of the plant. But during the greater part of the year 
the lake is extremely rich in plankton which acts as a screen and 
prevents the intense light from reaching the floor. It is only* 
during April and May that the water is transparent and so phyto¬ 
synthetic activity is increased sufficiently to float the balls. 
It is possible that the Aegagropilas are more widely distributed 
than is supposed, for their primary condition is a felt-like mass of 
isolated individuals living where the water is a few metres in depth. 
This can only be found by dragging. It is only under very special 
conditions that the absolutely regular ball forms are produced as 
instanced by the regular periodic occurrence of balls of Aeg' 
Sauteri at the surface of Lac Soro but not elsewhere, although the 
plant has been collected by dragging from other lakes. The isolated 
individuals must come under the rolling action of the waves which 
means that they must find their way to shallower parts of the water 
either naturally or by accidental means. But since less depth of 
water usually means greater intensity of light, it is only when the 
intensity of the light is lowered, either by presence of plankton as 
in Lac Soro or by other means, that the balls can exist. 
1 Wesenberg-Lund and Brand have shown by experiments in the 
laboratory that the balls rise to the surface in the daytime when the illumination 
is strong and sink to the bottom in the evening and remain there on a dull day, 
i.e., the rise to the surface is due to active photosynthesis in strong sunlight. 
