ROBERT A. BUDINGTON. 



It seems perfectly clear that the differentiated tissue extends 

 nearer to the tip of the root in the thyroid treated line than it does 

 in the roots of the control line: or, in tangible figures, it extends 

 20 per cent, nearer the tip. 



DISCUSSION 



The profound and almost semi-mysterious control of metabo- 

 lism by thyroid constituents, expressing itself mainly in the 

 phenomena of growth, has attracted many workers to an exami- 

 nation of very varied phases of its possible influence. The 

 present study is but one of this sort; and certain aspects of the 

 facts noted may well receive further comment. 



As said above, the main point in mind has been to discover 

 whether thyroid components, if absorbed by elementary rapidly 

 growing plant tissues, will cause in them an accelerated dif- 

 ferentiation comparable with what has been repeatedly described 

 as a distinct effect of it when used as a food by, grafted into, or 

 introduced as an extract into Amphibian larvae. A primary 

 awkwardness immediately presents itself in such an attempt, for 

 the plant root here employed shows such limited specialization 

 as compared with the complexity of an incipient animal organ 

 such as an appendage. The periblem and plerome tissues are the 

 only real differentiations from the parenchyma in a young root, 

 and they show little character in their apical extremities, their 

 identification being essentially dependent on position only. 

 They are, however, bona fide differentiations, as truly as muscle, 

 bone, or nerve tissue. 



Furthermore, it would be folly to claim that the experimental 

 results here reported are unique to, specific for thyroid "feeding"; 

 there may be dozens of organic and inorganic compounds, so far 

 as the writer knows, which could produce similar or even greater 

 effects of the sort noted than do thyroid components. No 

 attempt has been made to discover or compile such a list. Potas- 

 sium biniodide has often been employed to offset the effect of 

 thyroid deficiency in children. This fact, however, does not at 

 all lessen one's interest in thyroid substances and their normal 

 physiological values throughout the life-time of the possessing 

 organism. 



It may also seem that the interpretation given the experimental 



