R. N. Salaman and J. W. Lesley 35 



plants was noticed to resemble the "prostrate" variety differing however in 

 one characteristic manner, viz., the terminal part of the flaccid stem instead 

 of lying flat on the ground turns boldly upward. As growth proceeds 

 the proximal part of the stem assumes the horizontal position. Ana- 

 tomically the stem exhibits the same structure as does the " prostrate " 

 and shows the same extensive deficiency of interfascicular secondary 

 growth. At present the evidence is too incomplete to establish the 

 genetic relationship of the " procumbent " type of haulm, though we have 

 reason to think that it is a pure recessive differing in but one factor 

 from the " upright." 



We may represent the case then as follows : The " upright " potato 

 has undergone at least two modifications. The first, affects the stem 

 structure and is seen in the "procumbent" type. The second modification 

 besides that of stem structure has led to a further change, possibly 

 of tropic sensibility, and results in the " prostrate " habit of growth. 

 Whether the tropic modification occurs without the anatomical we 

 cannot say, although we believe that the anatomical can exist apart 

 from the tropic as is shown in the "procumbent" type. It is to be 

 regretted that there are at present no crosses between the "procumbent" 

 and " prostrate " families. 



Certain considerations naturally arise in connexion with the fore- 

 going results. One question is whether the anatomical peculiarity of 

 the " prostrate " and " procumbent " plants has any physiological effect 

 which we can detect. The deficiency of phloem does not amount to 

 much as the medullary phloem is not afiected, but the deficiency of 

 xylem is very pronounced though the large vessels are well represented. 

 The water supply to the leaves might be reduced and so affect the 

 metabolic processes and indirectly tuber formation. Experience shows 

 that " prostrate " and " procumbent " plants give individual crops fully as 

 large as "uprights," indeed, in 1911, an exceptionally dry summer, the 

 " prostrates " suffered less from drought and gave remarkably good crops. 



Again the failure of the interfascicular cambium does not appear to 

 be associated with poor tuber formation or as far as we know with 

 defective cell division elsewhere in the plant, so that this remarkable 

 loss of the power to divide appears to be confined to the interfascicular 

 region of the aerial stem. 



Indeed no correlation between habit of growth, incidence of Phy- 

 tophthera blight or other characters has hitherto come to light. It is 

 an interesting fact that the pure " prostrate " families showed a most 

 remarkable uniformity not only in habit of growth but in other characters 



3—2 



