124 CHAPTER VIII 



Description and Classification. — Of the large number 

 of Solaniim species only six are tuber-bearing, and of these 

 S. tuberosum (the common or Irish potato) is the only one of 

 agricultural importance to-day. The roots are fibrous, the 

 tubers being formed at the ends of rhizomes. " When grow- 

 ing under natural conditions the tubers are relatively small, 

 close to the surface of the soil or may be upon it. The pro- 

 duction of small green tubers on the haulm in the axils of the 

 leaves of the potato is not infrequent, and affords an inter- 

 esting proof of the true morphological nature of the under- 

 ground shoots and tubers. This phenomenon follows injury 

 to the phloem in the lower part of the stem, which prevents 

 the downward flow of the elaborated sap." Seeds are produced 

 on the upright stems in smooth berries, perfectly globose and 

 under an inch in diameter. The varieties of cultivated 

 potatoes to-day vary in the formation of berries, and in some 

 cases, owing to the production of non-viable pollen and general 

 incompatability, and also adverse climatic conditions, some 

 strains rarely form berries. In some cases even the formation 

 of inflorescences is suppressed. Self -fertilisation rarely, if 

 ever, occurs. When sown the seeds produce plants bearing 

 small tubers the first year, which reach normal size in about 

 three years. Since potatoes are normally propagated vegeta- 

 tively their purity is comparatively constant. Unlike the 

 artichoke tubers, that of the potato never bears roots. The 

 view is held by some that the formation of tubers is associated 

 with the presence of certain fungi, and that in reality the 

 potato tuber is a gall produced by a micro-organism. 



Potatoes are classified' into agricultural groups according 

 to the colour and shape of the tuber, sprouts and flowers. 



Shape of Tuber. — Eound, oblong, and long, either 

 flattened or round. Those tapering at one end are said to 

 be spindled when compared with uniform potatoes. 



Colour of Skin. — This may be white, cream-white, flesh- 

 coloured, pink, rose, red, bluish, mottled and russet brown. 



Colour of Sprouts. — " The colour of sprouts is very 

 important in determining the main groups of varieties. The 

 colour is determined by germinating the potato in the dark, 

 and as soon as the sprouts appear, they are examined usually 

 with a magnifying glass. The sprout is tipped with minute 

 scales or leaflets, which may be either coloured or white. 

 Also the base of sprouts may be either coloured or white. The 

 usual colours are white, cream-white, pink rose, rose-lilac, 

 magenta, lilac, violet, or deep violet." — jNTontgomery.* 



