GROWTH 213 



Suggested Experiments. Construct from small bored-and-split corks, 

 held by rubber bands, a series of small pressure-jackets which will just fit 

 on the tips of strong roots growing in sphagnum moss; and ascertain what 

 tension, in grams, can just be overcome by the swelling root. 



Some ingenious experiments to the same general end, but upon a dif- 

 ferent plan, are described by Osterhout, 73. 



A somewhat important phase of the relations of osmotic 

 pressure to Growth concerns the relative tensions of the tissues 

 in young shoots, a subject which will be considered later, but 

 which may receive practical illustration as follows : 



Suggested Experiments. Select a young, stout, firm-growing inter- 

 node some 100 mm. long, and measure its exact length; then, with a sharp 

 knife, quickly strip off a band of epidermis of the full length, and immediately 

 measure it; then strip off the entire woody tissues in four or five cuts, and 

 measure a piece of this; finally measure also the core of pith. 



Also select the stoutest young internode available, and peel off a ring 

 of epidermis. Immediately replace it and note the alteration of size. Or 

 use a young branch and remove a ring of cortex down to the cambium. Also, 

 select a firm herbaceous internode; quickly split it lengthwise through the' 

 middle, and note the curvatures. 



The foregoing study will show the remarkable relative ten- 

 sions which bring young growing plants into a condition of un- 

 stable internal strain. This suggests that the direction of longi- 

 tudinal growth might be altered by very slight causes, and thus 

 raises a question for practical study. 



Do slender growing parts under ordinary conditions grow 

 straight in the line 0} their axes, or do they move about (circum- 

 nutate? 



This may be determined by arranging for frequent exact observa- 

 tions of the positions of such parts. But as any movements must be 

 slight, it is necessary to ensure their magnification. 



Experiment. Select a seedling or other shoot in rapid growth. 

 Prepare, by drawing out glass tubing in the Bunsen flame to capillary 

 fineness, a very slender straight glass filament; at one end of a piece 

 2 cm. long put a minute drop of black sealing-wax, and a little way 

 over the other end slip a paper circle 5 mm. in diameter, with a very 

 small black spot in the center, through which the filament passes; 

 attach this end by thick shellac dissolved in alcohol to the tip of the 

 stem of seedling or shoot, and hold it until set. Then attach a similar 

 filament to the tip of a young leaf standing horizontally. Then place 

 the plant, uniformly illuminated, with the tips of each of the filaments 



