92 



DICOTYLEDONS 



branch. The shoot has no roots at first and must, th"^refore, get 

 all its food from the supplies stored up in the tubeii-, and this 

 causes the tuber to shrivel up and die, as the food stored in it 

 is exhausted. The new shoots in their turn throw out roots and 

 other underground shoots, portions of the latter being filled with 

 starch and swelled up to form fresh tubers (hg. 87). As, in their 



Fig. 88. — Section through the outer portion of a Potato. 



A'. Cork-cells. St. Cells containing starch-grains (140 times 



enlarged). To the left a starch-grain showing its stratified 



structure (500 times enlarged). 



uncultivated state, the tubers of the Potato plant remain in the 

 ground and give rise to a large number of new plants, it is of 

 great advantage to the new generation that the tubers are pro- 

 duced at the ends of runners, and are thus removed to a distance 

 from the mother plant. 



We see now clearly that the potato-tuber is a store of food for 

 the new plant. This food consists mainly of starch, which is also 

 one of the principal food-sul)stances of man. As the potato- 

 tuber contains no noxious properties and is easily ol)tained in 

 large quantities, it has become one of the chief vegetables we 

 eat. It is now cultivated nearly all over the world, but does 

 not grow well in the tropics. 



2. Leaves, Flowers and Fruit. — The stem of the Potato i)lant' 

 does not, as a rule, grow higher than about one foot and a half. 

 Tlie leaves are large and interruptedly pinnate, the leaflets being 



