92 PLANT PHYSIOLOGY 



added to the leaf. This is an important subject which presents 

 a definite problem as follows: 



7s the formation oj starch in leaves accompanied by an increase 

 of substance? 



This may be determined by a comparison of dry weights (dry to 

 exclude the effects of varying quantities of water), of identical green 

 tissues before and after they have made starch. 



EXPERIMENT. Select a large plant with somewhat thin leaves 

 of rather firm texture, such as one of the three common Geraniums, 

 or Fuchsia, and keep it for a day in the dark, or two days if low tem- 

 peratures prevail. On a bright morning bring it into a good light,, 

 and, by aid of a leaf-area cutter, cut as large a number of discs as 

 possible, preferably at least 100, from the leaves, taking them alternately 

 from the soft parenchyma of the right and left halves, but leaving 

 ample tissue for an equal number to be cut from the same leaves later. 

 Suspend the discs for two or three minutes over boiling water in a 

 test-tube, in order that the steam may kill them and prevent loss by 

 respiration; then place them in a drying-oven. Two or three hours 

 later cut 100 discs from the same leaves, kill them, and place them in 

 the oven. Dry both sets, giving them at first a temperature of about 

 60, raised gradually to near 100, until they cease to lose weight, 

 which will require about two days. Weigh both sets to milligrams, 

 compare the results, and reduce to grams per square meter per hour. 



PRECAUTIONS. The sets of discs should be cut alternately from the two 

 sides of the leaves, and not, as usually recommended, from one side or the 

 other, since, as Miss ECKERSON has discovered, the leaves of any given plant 

 commonly have one side or the other prevailingly heavier. It is best to take 

 the largest possible number of discs, since the error from local variations 

 in thickness, etc., tends thus to disappear; but at the same time too large 

 a number should not be taken from the leaves lest it do them injury. It is, 

 however, a fact, readily shown by experiment, that the removal of the discs 

 does not seriously disturb the functioning of the leaves. Two, or at most 

 three, hours of exposure to light are better than a longer time, since beyond 

 the former period the accumulation usually becomes much slower, thus 

 reducing the average increase per hour. BROWN and ESCOMBE, in a paper 

 of 1005, cited below under Literature, claim that this method gives results 

 much too high, but of this I am not convinced. 



LEAF-AREA CUTTERS. These were introduced in principle by SACHS, 

 when he devised this method (Arbeiten des botanischen Instituts in Wiirz- 

 burg, 3, 1884, 19, and Gesammelte Abhandlungen, i, 372), but his way of 

 cutting areas with a knife by aid of a pattern was cumbersome. Using this 

 general method, however, I have developed an instrument permitting facile 

 and accurate work in this experiment. It is among my normal apparatus 

 (page 46), and, as improved considerably from the original form described 

 in the Botanical Gazette, 39, 1905, 150, is constructed as follows. 



