Permeability 149 
of growth of the tubes was found to be inversely proportional to the 
concentration of sucrose, but if the concentration of the sugar was 
too low, the pollen tubes burst (see above). Thus the greatest rate of 
continued growth occurred in a solution of about 20 per cent, sugar. 
Since growth depends on the swelling of the protoplasm, that is, on 
the capacity of the protoplast to absorb water from the external 
medium, the introduction of substances into the external medium 
which alter the imbibition capacity of the protoplast differently from 
the osmotic pressure should give a means of determining whether the 
absorption of water by the cell is due to imbibition by the protoplast 
on account of its colloidal nature, or on account of differences in 
osmotic pressure on two sides of a semi-permeable membrane. 
Lloyd found that acids (hydrochloric, nitric, acetic and citric) 
added to 20 per cent, sucrose, did not affect the growth rate when 
the concentration of acid was between N /51200 and N /3200 but that 
in higher concentrations, from iV/1600 to N /200 the growth rates 
were less and less with progressive increase in the concentration and 
always terminated by bursting. When, however, acetic acid was 
added to 40 per cent, sucrose the growth rate when the acid was 
present in a concentration of N /3200 was increased to about four 
times that of the control. With alkalies added to 20 per cent, 
sucrose growth was depressed when the alkali was present in a con¬ 
centration of iV/400, but was greater when the concentration of alkali 
was within the range N/ 8 oo to N/ 12800, the greatest rates being with 
alkali concentrations of about iV/1600 and N/ 3200. At the higher 
concentrations bursting of the tubes took place. 
It will be noticed that the concentrations of acid and alkali used 
are negligible in comparison with that of sugar employed and so will 
not exert any appreciable influence on the value of the osmotic 
pressure of the external solution. On the other hand, the observa¬ 
tions of many observers (for example, Pascheles, 1897; Wo. Ostwald, 
1905, 1906; M. H. Fischer, 1910; Procter, 1911, 1914; Loeb, 1918) 
have shown that the swelling of gelatine and other colloids is greatly 
influenced by the hydrogen-ion concentration of the external medium. 
Lloyd has himself made experiments on the influence of acidity and 
alkalinity on the rate of swelling of gelatine. He concludes that for 
all acids there is a concentration which induces a maximum rate of 
swelling and a concentration which depresses swelling to a rate less 
than that of pure water. Alkalies produce a similar effect but it is 
not yet certain whether the minimum swelling rates are less than 
those occurring with distilled water. 
