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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



or hybrid, and contains but one dose of the factor for uprightness, 

 whilst the potato which hes on the ground from its birth has no 

 upright factor in it at all. The result of breeding in the second 

 generation shows that the prone potato, when selfed, gives rise to 

 nothing but prones, the upright to nothing but uprights, but the hushy 

 splits up into uprights, bushies, and prones in the proportion of 

 1:2:1. 



The prone condition is entirely independent of any of the factors 

 which control the growth or other characters of the potato, but it has, 

 as we shall see later, a secondary effect on cropping. The anatomical 

 basis of the differentiation between the prone and the upright plant 

 is to be found in the different distribution of the mechanical supporting 

 tissue — i.e, the xylem, or wood, in the lowest region of the stem. In 

 the potato family in general there are three primary vascular bundles. 

 In Ihe prone plants these vascular bundles remain quite distinct and 

 disunited. In the uprights, on the other hand, the three main 

 bundles are found as before, but they are united by a ring of secondary, 

 or interfascicular wood, so that the whole stem is encased in a woody 

 sheath. Growers have doubtless rejected the prone condition as one 

 only too liable to render the plant the more acoessible to disease; 

 but were it possible to exclude disease, I think that a prone potato has 

 a great deal to be said for it. Its chief value is that in time of drought 

 it retains the moisture around the roots, and in 1911 it was astonishing 

 how moist the soil was under a prone potato and how utterly desiccated 

 under an upright. 



Stolons. — The stolons, or underground stems, which carry the 

 tubers, are of varied lengths. In certain wild species, such as 

 S. Commersonii and S. verrucosum, they may attain some six 'feet 

 or more. In the parent, *S'. etuherosum, they may be two or three 

 feet long; in the domestic varieties they are not usually more than 

 nine inches to a foot long, and they may be very much shorter. I 

 think that it is extremely probable that the length of the stolons is 

 controlled by specific factors, and I have been able to show that 

 whilst the very long stolon individuals breed true to great length and 

 the very short also breed true, the medium varieties, when selfed, 

 give rise to families whose stolon lengths vary from quite short to 

 greater or lesser length. It is obvious that a character such as this 

 does not allow of definite measurement. I am inclined to think that 

 there are probably two, or may be three, pairs of factors which control 

 the " stolon length." 



Disposition of Tubers. — Immediately related to, if not, indeed, 

 identical with, the subject of " stolon length " is that of the question 

 of the relations in space between the position of the tubers and the 

 central axis of the stem. It may be said at once that in long stolon 

 plants the tubers are correspondingly distant, and in very short stolon 

 plants the tubers are close up. 



The evidence for factors controlling this disposition of tubers is 

 practically the same as that which was found for the stolon formation : 



