[LLOYD] PROPERTIES OF PROTOPLASM 135 



The method used was as follows. 



Pollen grains of the sweet-pea {Lathyrus odoratus), which on 

 account of their shape and the manner of growth of the pollen-tube 

 allow clear cut observation and measurement, were grown in hanging 

 drops. The cover-glass carrying the drop-culture, was supported by a 

 glass ring, and evaporation prevented by the addition of a drop of the 

 culture medium on the bottom of the cell. Each series was run in 

 duplicate. Measurements of the length of the growing pollen-tubes 

 were made with an eye-piece micrometer with a low power (24 mm.) 

 objective. The unit used was equal to 14 microns. As few grains 

 as possible were used in each drop, being placed there with a needle- 

 point. It is convenient to scatter the pollen evenly on a glass plate, 

 from which it is scraped with the needle-point and placed on the 

 cover-glass prior to adding the culture medium. It was found that 

 the pollen grains, becoming wet seriatim, do not begin growth evenly. 

 For this reason the maximum growth was taken, the duplicate series 

 serving for control. Aberrant individuals were disregarded, or 

 recorded as such. It is unnecessary to add that there are plenty of 

 sources of error, all of which so far as recognized, were controlled. 



Experiments with cane-sugar . It was first necessary to obtain 

 a control culture medium from which acids, salts and alkalis were 

 absent. It was assumed that cane-sugar c.p. in distilled water would 

 serve. As the accompanying graph shows, the initial growth rate 

 is most rapid in the solution of lowest osmotic pressure used (10%), 

 in that in which the greatest amount of swelling can take place in 

 the shortest time. But a length of tube greater than 6 units cannot 

 be attained because when (or before) this is reached, the protoplasm 

 swells sufficiently to burst the tube. So far as we can see, this is the 

 only limiting factor involved. In a 20% solution growth was main- 

 tained for 20 hours at least, with no bursting, the highest growth rates 

 occurring after the tubes were 8 units long. This is the time approxi- 

 mately when vacuolization begins. The initial rate in the higher con- 

 centrations used were successively lower, as were also the subsequent 

 rates. The maximum amount of growth in the 50% solution was al- 

 ways very small, but actual and measurable. In these concentrations 

 (30 — 50%) the tubes at length tend strongly to become spherical — 

 in actuality they become bulbous or clavate — before the supervention 

 of vacuolization, a fact which, it is admitted, appears to be out of 

 harmony with the general teaching of the experiments, for the less 

 the imbibition pressure of the protoplast, the less tendency towards 

 sphericity, it may be argued. This aside for the present, it is seen 

 that pollen tubes growing in 20% cane sugar give us, in both rate and 

 total amount, a basis for comparison by which we may test the effects 



