24 Genetic Studies in Potatoes 



of relatively small parenchymatous cells, towards the centre the cells are 

 large with intercellular spaces. The medullary phloem occurs in little 

 strands, especially in the small-celled pith near the protoxylem. These 

 phloem strands originate in the medulla independently of the cambium, 

 but Artsch wager's ^ work has shown the interesting fact that these 

 strands anastomose and at certain points are actually continuous with 

 the ordinary outer phloem so that all the phloem of the potato plant 

 forms one system. It may be added that cells containing fine crystal 

 sand occur abundantly in the pith and to a less extent in the cortex. 



Thus we see that the predominating mechanical tissue of the normal 

 stem is the xylem and to a lesser extent the fibres which occur near 

 the phloem and the collenchyma of the cortex. If there be any anatomical 

 peculiarity of the stem affecting its rigidity and power to maintain an 

 erect habit, it is to the xylem that we should first look. 



A histological examination of the stems in the abnormal family 

 grown at Cambridge^ and of its descendants has brought a very definite 

 peculiarity to light (Plate IV, fig. 2). The part of the adult stem 

 taken was the internode of the basal part next to the ground, and the 

 stains chiefly employed were Safranin, Phloroglucin and Schultze's 

 solution. All thirty seedlings were examined and the following obser- 

 vations apply to them in common. While the fascicular cambium had 

 formed much secondary phloem and xylem, in the interfascicular region 

 or medullary rays between the bundles secondary tissues were either 

 deficient or entirely wanting, and as the great bulk of the secondary 

 tissue is xylem, the defective development of this tissue was most 

 conspicuous. 



Whilst the thick-walled lignified xylem of a normal stem forms 

 a complete cylinder which may sometimes be thin or even broken 

 for a very short space immediately below the insertion of a leaf, in the 

 " prostrate '•' stem nothing approaching a continuous cylinder of wood is 

 formed. The contrast is strikingly seen in pieces of stem which have 

 decayed in water, or again after treatment with strong nitric acid so 

 that all the tissues disappear except the lignified xylem (Plate V, 

 fig. 1). In the normal stem the xylem forms a continuous system 

 made up of fascicular groups connected by broad interfascicular bands, 

 forming thus in transverse section a continuous ring of woody tissue ; 

 in the "prostrate," on the other hand, the wood rarely forms a con- 

 tinuous ring, and then only at isolated points in the length of the stem. 



1 Artschwaper, J. Agr. Res. 1918, Vol. xiv. No. 6. 



2 The histological work in this paper has been done by J. W. Lesley. 



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