354 PLANT PHYSIOLOGY 



of Bose. It would seem possible, however, that this " nervous" 

 reaction measured by Bose and Molisch is the rapid or " explosive" 

 conduction of Ball. Probably the stimulus travels mostly in the 

 pith and xylem in the stem, as stated by Snow, and changes over 

 to the phloem in the petioles, where the hormones in question are 

 aided by the same diffusion processes of Mason and Maskell 

 (Chap. XXIII) so useful in the translocation of foods. Thus we 

 are able to harmonize to some degree the contributions of these 

 various workers. 



Polarity and Correlation. — In Chapter XVII, attention was 

 called to the fact that in stems one end is associated with the 

 formation of leaves and branches while the other end forms only 

 roots. This tendency to form organs of a distinct kind at the 

 various ends or parts, is described by saying that the organism 

 shows polarity; this is not an explanation but merely a restate- 

 ment of the phenomenon. After the polarity has once been es- 

 tablished it is generally difficult to change, although in some forms 

 like Bryopsis (one of the Algae) the polarity may be reversed 

 after it has once been determined. In Chapter XVII these phe- 

 nomena were explained as due to diffusing hormones, but other 

 explanations, in some cases more concrete, have been offered. 

 Thus Davies and Hicks have shown that in the case of willows, 

 which develop shoots from the upper end and roots from the 

 lower, this polarity is connected with the ratio of the carbohy- 

 drates to the nitrogen compounds (the C:N ratio). The shoots 

 tend to form where there is the largest amount of nitrogen, i. e., 

 where the C/N ratio is lowest, and roots from places where there 

 is less nitrogen and more carbon compounds. 



By correlation is meant the reciprocal influence of organs. In 

 addition to the examples cited in Chapter XVII, a few others may 

 here be mentioned. Among the frequently observed quantitative 

 correlations is the relation between blossoms and vegetative parts 

 in the sweet pea (Lathyrus). As long as the flowers are removed, 

 new buds and blossoms are produced; otherwise blossoming stops 

 and fruiting begins. Fern gametophytes live ordinarily only un- 

 til the sporophyte is well established, but if fertilization is pre- 

 vented, the death of the prothallus may be delayed for months, 

 with the production of new archegonia. The production of blos- 

 soms and fruit on fruit trees may be increased in this same fashion 

 by judicious pruning. 



