INTRODUCTION 257 



to attribute the various types of form seen in leaves to differences in these 

 materials. But even if that were possible, we must still, as in mineralogy, treat 

 of the form of a plant by itself ; and even though we may be able to prove that 

 a definite form results from the presence of a definite material constitution still 

 we do not know why it is so ; why, for example, calcium oxalate crystallizes in 

 a tetragonal form when three molecules of water are present, and in the mono- 

 symmetrical form when there is only one, Nowadays, when an explanation of 

 form as due to chemical constitution is still quite impossible, it appears to us 

 that a special section dealing with ' metamorphosis ' is essential. 



It is, perhaps, not possible to sharply separate off change of form 

 either from the second department of physiology, the transformation of energy 

 more especially the phenomena of movement or from morphology. How- 

 ever, the delimitation is of service from the practical point of view, since a large 

 series of vital phenomena, especially those connected with reproduction, heredity, 

 evolution of species, &c., may be most naturally classed along with other forma- 

 tive processes, while they cannot be treated of in a binary classification of the 

 subject nor be dealt with in an appendix. This, however, is of no consequence 

 so far as the plant is concerned. 



In the following pages we have to investigate first of all how growth and 

 formation is carried out under constant external conditions (Lectures XXI- 

 XXIII) ; we shall thus learn to appreciate the mode of action of internal causes 

 of growth, although we shall not gain thereby any closer insight into their 

 nature ; at the same time we shall endeavour to form a conception of what 

 growth and formation really are. Thereafter we will devote ourselves to the 

 study of the influence of the most important external factors on growth 

 (Lectures XXIV-XXVI). In conclusion we will devote ourselves to the study 

 of the developmental life- cycle itself, which results from the simultaneous opera- 

 tion of both internal and external factors. 



Bibliography to Lecture XX. 



ALBRECHT. 1901 Biolog. Centrbl. 21, 97. 



ASKENASY. 1870. Bot. Ztg. 28, 193. 



BERNARD, CL. 1878. Lecons sur les phenomenes de la vie etc. Paris. 



BERTHOLD. 1 898. Unters. zur Physiologic der pflanzl. Organisation, i . Leipzig. 



BUTSCHLI. 1901. Mechanismus u. Vitalismus. Leipzig. 



CLAUSSEN, J. 1901. Jahrb. d. hamburg. wiss. Anstalten, 18. 



DRIESCH. 1901. Die organischen Regulationen. Leipzig. 



[DRIESCH. 1905. Der Vitalismus als Geschichte und als Lehre. Leipzig.] 



GOEBEL. 1889-93. Pflanzenbiolog. Schild. Marburg. 



HERTWIG. 1897. Mechanik u. Biologic. Jena. 



HERTWIG. 1898. Die Zelle u. die Gewebe, Vol. 2, Chap. 5. Jena. 



[HERTWIG. 1906. Allg. Biologic. Jena. (Cited by Wolff, 2nd ed. 1905.)] 



KERNER. 1891. Pflanzenleben. Leipzig and Vienna. 



NAGELI. 1860. Beitr. z. wiss. Botanik, 2, 46. 



PFEFFER. 1893. Die Reizbarkeit der Pflanzen. (Verhandl. d. Gesell. deut. 



Naturf. u. Aerzte.) 

 RACIBORSKI. 1896. Flora, 82, 107. 



REINKE. 1901. Einl. in die theoretische Biologic. Berlin. 

 SACHS. 1880. StofiE u. Form der Pflanzenorgane. Arb. Wiirzburger Instit. 3, 452. 



(Ges. Abh. 2, 1159.) 



SCHENCK, H. 1886. Biologic d. Wassergewachse. Bonn. 

 SCHWENDENER. 1874. Das mechanische Prinzip im anat. Bau d. Monocotylen. 



Leipzig. 



VOLKENS. 1887. Flora d. agypt.-arab. Wiiste. Berlin. 

 WOLFF, G. 1902. Mechanismus u. Vitalismus. Leipzig. 



JOST S 



