158 HYDROPHYTES sect, iv 



the extent of its surface and its precise shape, which constitute the factor 

 known as ' form-resistance ', and partly on the buoyancy (specific gravity 

 and viscosity) of the surrounding hquid. Ostwald conchides that 



The rate of sinking 



Excess of weight (of the organism over that of an equal volume of water) 



Form-resistance x viscosity (of the liquid). 

 If therefore a body is to float, or, in other words, if its rate of sinking is to 

 be reduced to a minimum, the numerator (excess of weight) must be de- 

 creased, and the denominator increased as much as possible ; so as to 

 make the quotient approximate to zero. Hence, in order to decrease their 

 rate of sinking, plankton-organisms in the first place endeavour to decrease 

 the excess of weight. This is effected by the nature of the cell-contents 

 (products of metabolism, such as fat and gases, play a part), also by the 

 thickness of the cell-wall (which is always extremely thin), and it must 

 necessarily be different in species belonging to salt water and fresh 

 water respectively. Plankton-diatoms have thinner walls than have 

 those living on the bed of the water. It should be specially noted that 

 fatty oil supplies an effective means of flotation, and that diatoms 

 manufacture oil ; this fact sheds light upon the universal and great 

 part played by diatoms in plankton .^ Fat is also manufactured by other 

 plankton-organisms, such as Flagellata. 



Flotation-devices. Again, the organism strives to increase the ' form- 

 resistance ' as much as possible, by relative increase of surface due to 

 decrease in volume, and an absolute increase in surface due to deviation 

 from the spherical form. To obtain a shape presenting the maximum 

 vertical projection (transverse section), the flotation-apparatus appears 

 always to be placed horizontally, and therefore at right angles to the 

 direction of sinking. Schiitt ^ has demonstrated several arrangements 

 designed to enlarge the surface of microscopic plankton-organisms, which 

 thus acquire an increased power of flotation and of evading too sudden 

 ascents or descents. Such sudden movements tend to be caused by 

 changes in the physical characters of sea-water, and may endanger the 

 lives of the organisms. Plankton-organisms, and particularly diatoms, 

 are, relatively speaking, extraordinarily large in surface : in some 

 diatoms and Peridineae the surface is increased by a flotation-apparatus 

 in the form of wing-like extensions, threads, bristles, and spines ; or the 

 body itself is as a whole filiform, sometimes curved or spirally coiled, 

 as in some diatoms ; others are helmet-shaped, or parachute-like, or 

 possess sail-like or annular processes ; still others are combined into 

 threads, or into gelatinous masses. All these features remain inexplic- 

 able, except on the theory here given, though in some cases spines may 

 serve as means of defence against foes. Temporary changes in the form 

 of the flotation-apparatus are known to occur.^ 



The preceding argument is strengthened by a comparison between 

 plankton-diatoms and ground-diatoms. The latter are fixed or creep 

 about, and possess in their shells sutures through which the protoplasm 

 projects, so that they can move about, seek the most favourably illumi- 

 nated spots, and fix themselves there. The majority of plankton-diatoms 



* Beijerinck, 1895. " Schiitt, 1893, 



* Wesenberg-Lund, 1908, and others. 



11 



