FUNCTION.] ANOMALIES IN SECRETIONS. 303 



for eliciting the chemical action which ends in producing 

 poison, although it may be high enough for mere vegetation. 

 The inferior quality of European Rhubarb, Hemp, and Tobacco 

 is perhaps referable to the same circumstance. 



The quality of secretions is not, however, always dependent 

 upon the action of external agents : but is in some cases con- 

 nected with vital processes too obscure for the explanation of 

 the physiologist. Dr. Christison has found by experiment that, 

 although the Midsummer leaves of Conium are very active, 

 yet that they are eminently energetic in the young plant^ 

 both at the beginning of November and in the month of March, 

 before vegetation starts on the approach of genial weather. 

 The fruit, in like manner, is most active when full grown, but 

 still green and juicy ; it then yields much more of the active 

 principle Conia than afterwards, when it is ripe and dry. 

 Possibly this may have some connection with the chemical 

 composition of Conia, which being an azotised principle may 

 be expected to disappear with the nitrogen necessary to its 

 constitution, and which is most abundant in the youngest 

 parts. In like manner Tannin seems to be more abundant 

 in young plants than in old, in spring than in autumn, 

 although, not being an azotised product, the conjecture as 

 to Conia will not apply to it. 



In many cases a secondary action takes place in plants, 

 dependent upon mere vital force, and unconnected with any 

 direct action of light, air, or temperature. There can be no 

 doubt that the process of " ripening " in the Potato, which 

 consists in a gradual change of its gummy matter into starch, 

 is of this nature. This phenomenon occurs in the tubers, is 

 naturally produced in darkness, and in a low temperature, 

 and is independent of light. It goes on till late in the 

 autumn, and does not cease till the temperature becomes too 

 low to favour chemical alterations. Schleiden has, however, 

 objected to this statement. " I must here advert to a mistake 

 which has often been repeated, and which if continued will 

 throw Vegetable Physiology into confusion. De Candolle 

 thought he had shown that lOOlbs. of Potatoes gave in August 

 10 Ibs. of starch, in September 1H Its., in October 14f Ibs., 

 in November 17 Ibs., in April 13^ Ibs., in May again 10 Ibs. 



