INTRODUCTION 3 



the bending of the peduncle and that if we remove the bud the lesion itself 

 induces the straightening of the peduncle. This experiment also teaches us 

 to be on our guard against introducing, during the course of a physiological 

 experiment, any new factor hkely to produce special reactions on its own 

 account. 



A discussion of plant metabolism presupposes a knowledge of the chemical 

 composition of the plant, and to this subject we must therefore first of all turn 

 our attention. 



Qualitative analysis discloses the existence in the plant body of a relatively 

 small number of elements. Ignoring such as are found only in certain plants, 

 such as are present only when artificially supphed, such as occur only in 

 minute quantities, and which are obviously of no importance to the plant, there 

 are left thirteen elements for consideration, viz. hydrogen, oxygen, chlorine, 

 sulphur, nitrogen, phosphorus, sihcon, carbon, potassium, sodium, calcium, 

 magnesium, and iron. The gain in scientific insight which such an analysis 

 affords us is, however, very limited. 



Nor does quantitative analysis give us a much deeper insight into the 

 subject, although the following table, borrowed from Ebermayer (1882, p. 47), 

 may prove useful as indicating the amount of carbon, hydrogen, oxygen, 

 nitrogen, and mineral matter present in 100 parts of plant substance, dried 

 at a temperature of 100° C. : — 



C. H. O. N. Ash. 



Wheat grains ... 46. i 58 43.4 23 2-4 



Oats /0.7 6-4 36.7 2.2 40 



Rye straw .... 49.9 5-6 406 o-j 3-6 



Potatoes 440 5-8 44-7 i-S 40 



Peas 465 6-2 40-0 4-2 3-1 



Leaves of Red Beet . 38.1 5-1 308 4-j 21J 



On the other hand, information as to the chemical compounds occurring 

 in plants is of much greater value. The number of these is, however, so enor- 

 mous that it is out of the question to attempt an exhaustive enumeration of 

 them ; moreover, investigations as to their nature are as yet far from being 

 complete. We know, for example, of the existence of one or more compounds 

 in certain species only, while there are many which appear to be characteristic 

 of genera or famihes. The majority of these substances are hy-products of 

 metabolism and have on that account scarcely received from physiologists the 

 attention they deserve. Putting on one side the inorganic compounds for the 

 most part absorbed from the environment, and also the organic substances above 

 mentioned which are of Hmited distribution, we have left for consideration a large 

 number of organic compounds common to all plants. These are compounds 

 of carbon with one or more of the elements, hydrogen, oxygen, nitrogen, sulphur, 

 and phosphorus. It will be convenient at this point to present a brief sum- 

 mary of the more important of these bodies, classifying them rather according 

 to their physiological value than their chemical constitution. It is impossible 

 at the present stage to enter into a discussion of their chemical characters, 

 although later on it may be necessary in special cases to give some details on 

 these points. For all general questions of chemical constitution and pecu- 

 liarities we must refer to the standard text-books on Chemistry and on Physio- 

 logical Chemistry, such as those of Ebermayer (1882), Hammersten (1899), 

 and FuRTH (1903). [Special reference must also be made to the recently 

 published treatise by Czapek (Biochemie der Pflanzen, Jena, 1905. 2 vols.). 

 It deals in a remarkably thorough manner with all the more important chemical 

 problems in Plant Physiology and constitutes a handbook quite indispensable 

 to the student of Biology. J 



B 2 



