EFFECT OF EXTEENAL AGENCIES ON GKOWTH. 603 



piadacese, &c. (Warming) ; but the distinction between lateral bud 

 formation and terminal dichotomy is bridged over by numerous in- 

 termediate conditions. 



From what has been said on the morphology, structure, and 

 mode of development of the several organs, it will be seen that all 

 the organs of Flowering plants may be reduced to two types 

 that of the axis (caulome) and that of the leaf (phyllome), with the 

 hair-like projection from the epidermis (trichome). But no arbitrary 

 line, applicable in all cases, can be drawn between these, as they 

 all merge one into the other, or are connected by intermediate gra- 

 dations. The distinguishing character between stem and leaf least 

 liable to exception consists in the circumstance that a bud may be 

 and is constantly developed at the apex of the stem, while it next 

 to never is at the extremity of the blade of the leaf, though it may 

 be at the end of the petiole. 



CHAPTER IV. 



GENERAL PHENOMENA CONNECTED WITH GROWTH. 



In preceding Sections a general view has been taken of the anatomical 

 appearances and physiological phenomena presented by plants during their 

 growth and development. It will have been seen that the physiological 

 processes alluded to are most intimately associated with, and dependent 

 on, chemical and physical changes, for a full account of which reference 

 must be made to chemical and physical text-books. While the ordinary 

 student has, in most cases, abundant facilities for studying morphology, 

 minute anatomy, and systematic botany, he has rarely the opportunity of 

 studying for himself, otherwise than in books, the details relating to the 

 chemistry and physics of vegetation ; and in not one of our schools and 

 colleges is any adequate pro vision yet afforded for this purpose. The account 

 which can be given here must necessarily be of a very meagre character. 

 The student who wishes to pursue the subject further should read the 

 third and fourth chapters of the third Book of the English edition of 

 Sachs's ' Text-book.' 



Effect of External Agencies on Growth, fyc. 



A plant as a living organism, existing, feeding, breathing, grow- 

 ing, and reproducing itself, is necessarily dependent on many con- 

 ditions, such as exposure to the atmosphere, access to light, to 

 food, &c. It is under the influence of light, of heat, of electrical 

 currents, of gravitation, &c. And these agencies do not exert their 

 influence singly, but in combination. The effect of the one is 

 modified by that of the others. The living plant is never entirely 



