NUTRITION. THE MACHINERY. 37 



it is proyided with " eyes," which eyes, as may be seen 

 when the tuber begins to sprout, are nothing but buds. A 

 tuber, then, is- a portion of the stem of the plant, in which 

 the tissues become thickened and filled with nutritive matter 

 (in this case starch), which is provided with buds, and 

 which, when once fully formed, is separated from the 

 parent haulm or stem by the gradual decay and death of 

 the latter. A tuber of this kind fulfils in the economy of 

 the plant much the same purpose as the seed ; and hence 

 the term " seed potato," though far from correct in a 

 technical sense, conveys, nevertheless, a not wholly in- 

 correct idea. 



A "bulb," such as that of an onion, is a portion of the stem 

 modified for the same purposes as the tuber ; but whereas 

 in a tuber the stem itself is swollen, and the leaves reduced 

 to the merest scales, in a bulb the conditions are reversed : 

 the fleshy scales of an onion bulb are really the bases of the 

 leaves, as anyone may see who will examine an onion in 

 growth, while the stem itself is reduced to a mere flat and 

 not very thick plate, from the sides of which emerge the 

 leaves. The term bulb as applied to such a root as the 

 turnip is inaccurate. 



These illustrations, taken from plants most familiar to 

 the cultivator, will suffice to show the general character of 

 the stem and its subdivisions, and will indicate the great 

 extent of variation there is in its outward characteristics. 

 The inward conformation of the stem varies according to the 

 nature of the plant its age and the purpose it fulfils. 

 The structure of the stem of a timber tree and that of a 

 potato tuber both, as we have seen, forms of stems are 

 naturally different. In the one case thin- walled cells filled 

 with starch predominate, in the other wood-cells and fibres 



