STRUCTURE OF PROTOPLASM 4r 



morphological classification are directly evident to the observer, whereas the appear- 

 ance of an organ does not always afford direct or trustworthy evidence of its 

 functional value. Indeed, an organ may commonly perform more than one 

 function, and in accordance with prevailing external conditions it may result that 

 for a time a particular function is most in evidence, or even the only active one. 

 Hence in a classification according to function, the same organ, or the same cell, 

 would at one time come under one, and at another under a quite different, 

 category. 



We may, however, speak from a physiological point of view of assimilatory, 

 translocatory, and mechanical systems, for morphology deals only with the shape, 

 position, &c. of the functional parts, and it is evident that organs of the same 

 morphological value may have very different tasks to perform, and vice versa. 



In the affairs of everyday life similar instances are to be found. Thus, 

 railways would naturally be classified according to the districts through which they 

 run and the companies to which they belong, though the purposes for which the 

 passengers travel and for which the goods are sent, as well as the uses which both 

 afterwards serve, are of the highest importance and interest. The same line of 

 rails may serve at one time for goods and at another for passenger traffic, or even 

 may be used by the trollies on which navvies travel from place to place. Here, 

 just as in a living organism, similar shape and structure does not necessitate an 

 equivalent functional importance. 



SECTION 7. Structure of Protoplasm. 



It has previously (Sect, i) been stated that Physiology must neces- 

 sarily seek an explanation of all vital processes in the developmental and 

 formative powers of the protoplast. Our knowledge on this point is still 

 in its infancy, and we must be content if we can gain here and there 

 a glimpse into the internal protoplasmic mechanism. Even though our 

 knowledge with regard to the structure of protoplasm were to be- 

 enormously increased, we should still see, not the causes and forces which 

 are acting, but only the results which they produce. The most perfect 

 mental picture of the plant or of the protoplast must necessarily fail to 

 reveal the hidden and invisible causes which make it assume its specific 

 form. 



In the different sections of this book, the visible processes taking place 

 in the protoplast will be dealt with so far as an accurate physiological 

 knowledge of them has been obtained, and will be traced as far as possible 

 to their ultimate causes and origin. 



A complete morphological knowledge of the shape and structure of 

 the protoplast on the part of the reader is assumed 1 . The following 



1 See O. Hertwig, Die Zelle u. d. Gewebe, 1893 ; the literature collated by Zimmermann, 

 Beihefte zum Botan. Centralbl., Bd. in, pp. 206, 321, 401 ; 1894, Bd. iv, p. Si ; and the numerous 

 recent special works of Strasburger, Rosen, Bovin, Hertwig, Waldeyer, &c. 



