MOVEMENTS OF IRRITATION. 525 



It may farther be remarked that on treatment of the sections 

 with phloroglucin and Hydrochloric acid, the sclerenchyma and 

 bast fibres of the mechanical tissue, in all cases which I examined, 

 stained red (see 42), and must therefore have been lignified. 



1 See also the memoirs cited at the end of 195, and especially Schwendener's 

 work. 



197. Correlation. 



The growth of one part of a plant frequently exerts a certain 

 influence on that of another part of the same individual. We have 

 in recent times begun to devote special attention to the facts re- 

 lating to correlation in the vegetable kingdom, and we will now 

 acquaint ourselves with some of these facts. 



If we deprive young fir plants (Abies excelsa) of their apical 

 shoot, it will be found that, in the course of one to three years, 

 one or more of the horizonal lateral shoots of the uppermost 

 whorl elevate themselves. One of these generally obtains the 

 upper hand; it then completely replaces the terminal shoot 

 removed. This is seen not only in its orthotropism, but also in the 

 arrangement of its branches. A horizontal lateral shoot of the fir 

 branches chiefly in & horizontal direction, right and left, while a 

 normal terminal shoot or one of these substituted lateral shoots 

 forms four- or five-rayed whorls of branches. In the-experiments 

 which I made to acquaint myself with the phenomena of correla- 

 tion here under consideration, I used fir plants growing in the 

 woods, about the height of a man. 1 



If we place potatoes in a dark place with their morphological 

 base downwards (the tubers need not be laid in soil, and we need 

 not even supply them with water), we shall find that after a 

 shorter or longer time scarcely any buds sprout except those 

 situated near the morphological apex. If, however, in some of 

 the potatoes we now remove the shoots developing at their apex 

 as they appear, it is seen that this operation induces the de- 

 velopment of more basally situated buds, which would not have 

 developed, or only to a slight extent, if we had not broken off the 

 shoots appearing at the apex. 



A further instance of correlation can easily be demonstrated in 



