106 PLANT PHYSIOLOGY. 



by Barnes in the same volume, 150 ; another is described by Stone, Bo- 

 tanical Gazette, 17, 105, and another in the same journal, 22, 258; another 

 is given by Golden, Botanical Gazette, 19, 113, (for growth in thickness,) 

 and another by Arthur, Botanical Gazette, 22, 463. Corbett has described 

 another form in the Ninth and Twelfth Annual Reports of the West 

 Virginia Agricultural Experiment Station. 



66. By what mode of expansion (in what parts) do leaf, root, 

 and stem increase in size ? 



Answer by Experiments 34, 35, 36. 



EXPERIMENT 34. For leaves this may be tested by marking very 

 young ones into regular areas and observing the changes in form and 

 size of those areas during growth. With a rubber stamp marking 

 small squares, stamp the upper surfaces of very young leaves, holding 

 them against a cork to secure a firm flat surface, and observe subse- 

 quent changes in the markings. 



Of course the stamp must be made to order, but it is inexpensive ; one 

 marking 45 (9 x 5) squares of 3 mm. on a side is excellent ; the lines must 

 be fine ; a black stamp-ink should be used, and water not allowed to 

 touch the leaves. 



The same result may be obtained by pushing pins at regular inter- 

 vals through a piece of sheet cork, with points projecting a little, and 

 pressing these points through the leaf, which is held against another piece 

 of cork. The spread of the holes will give a record of expansion of the 

 leaf, but not so effectively as in the case of the stam-p. Regularity in po- 

 sition of the pins may be secured by putting them through the angles of 

 a piece of cross-section or plotting-paper placed on the cork. 



EXPERIMENT 35. For stems this may be answered by marking 

 very young ones at short and regular (about 2 mm.) intervals from the 

 tip backwards by water-proof India ink applied with a stretched 

 thread (see page 39), and observing their rate of separation. Stems 

 as free from leaves as possible should be selected. 



EXPERIMENT 36. For roots this may be answered after the prin- 

 ciple used for stems. Start a few horse-beans in wet sphagnum, and 

 when the roots have developed to the length of an inch, mark one or 

 more of them, at intervals of 2 mm. from the tip backwards, with 

 water-proof India ink applied by a stretched thread. Fasten the seed 

 to the cork of a flower-pot moist chamber (see page 1 18) with the root 

 pointing directly downwards, and after a day or two note the posi- 

 tions of the marks. 



The roots may also be grown in thistle-tubes, as sometimes recom- 

 mended, on the outside of which the absolute growth may also be recorded, 

 but in my experience the method is inferior to that described above. 



