Forms of Stems 



57 



g. The tuber is an underground stem or branch, which is 

 much swollen by the deposition of a large quantity of starch 

 and other food materials. It posseses leaves which are more 

 or less modified. In the Potato (which is a good example of 

 the tuber) the eyes are the modified leaf buds. It is a well- 

 known fact that if the aerial branches of a Potato plant are 

 earthed, their growth will be arrested and tubers will be formed. 



Fig. 84. — A six -weeks-old Potato plant, developed Fig. 85.— Single tuber of the 

 from the seed, the upper branches, a, b, being Pig Nut (Canitfl bulbocas- 



cut off: d^ cotyledons ; in the axils of the cotyle- ta-nuvi). 



dons are developed the underground branches, f, 

 e, which penetrate into the ground and form 

 tubers, _/^^, at their apex or in the axils of small 

 leaves. The tubers are formed only on the 

 branches which are produced in the axils of the 

 cotyledons, never on the true roots h. 



Other examples of tubers are to be seen in the Jerusalem 

 Artichoke and in the Pig Nut. 



The two following modifications of stem are only to be met 

 with amongst monocotyledonous plants. 



h. The bulb consists of a more or less flattened disc-like 

 stem, giving off roots below, and scale-like leaves, together 

 with stem and flowers, above. 



The scale-like leaves have the power of developing in their 



