164 



water increases the cell pressure, especially in the young- eye cells with their 

 still elastic walls, so that the eye begins to grow. Young shoots sprout from 

 these eyes ultimately reaching the upper surface of the soil. This more un- 

 usual condition occurs only after continued wet weather. As a rule, only 

 passing periods of rain force the water into the tubers, an effect lasting but 

 a short time ; then the sprout remains short and thickens to the secondary 

 tuber (Kindel). 



The cork layer (the skin, smooth in young tubers) shows very clearly 

 how the cells of the ripening tubers lose their elasticity. When the tubers 

 are very ripe the skin becomes rough in most varieties of potato, especially 

 red ones. At first the cells of the cork layer are closely connected with one 

 another but, with the increasing pressure of the swelling parenchyma, the 

 cells are forced apart, tearing the skin. Under these tears new cork cells 

 are formed. This splitting of the skin is greater or less with different 

 varieties. The more split a tuber of an otherwise smooth-skinned variety 

 is, the riper it is and the richer in starch. 



Diaphysis of the tubers in many cases has a bad influence in that the 

 quantity of starch which may be regarded as influenced by the soil, is de- 

 posited in a less available form than in normal development. Together with 

 the large tubers a great many small ones are formed, which are less mature 

 and therefore poorer in starch. According to the investigations of Kiihn' 

 and Weidner-, the tubers already present do not become poorer in starch 

 when the secondary tubers are formed. The starch of the secondary tubers 

 does not come from the original tuber, but directly from the leaves. Only in 

 plants, whose foliage is dead, does a suddenly renewed supply of water pro- 

 duce secondary tubers at the expense of the starch content of the older ones. 

 Both old and young tubers have only the starch content of the healthy 

 tuber, which has not grown out. 



.'^o-called "water ends" are nothing but the result of a renewed growth 

 of the apical jiarts of the tuber excited by a subsequent supply of water. 

 These are thereby lengthened into a conical form and are filled with new 

 starch (see the right side of Fig. 17). The starch filling is just as scanty 

 as in the real sccondar}^ tuber, "Kindel." 



FoRM,\TION OF TuH1:RS ^^'lTHOUT FoLTAdE. 



If tubers, at the time they would sprout naturally, are not put in the 

 earth, l)ut are kept in a dry, poorly lighted room until the next period of 

 harvesting, a number of small tubers will sometimes begin growth. These 

 stand either close against the mother tuber or hang from short stolons, 

 which have developed from the eyes. AMiile, with a timely sup[)ly of water 

 and light, these eyes would have grown into leaved, green sprouts, in the 

 dry. dark store-room, the sprouting eye has developed into a thread-like 

 runner (stolon) beset with scales instead of leaves, the tip of which has 

 thickened into a tuber. 



1 Zeitschr. d. landw. Centralvcr. tier Prov. Saehsen 1868, p. .322. 



2 Annalen des Mecklenb. patriot. Ver. 1868, No. 39. 



