NUTRITION. THE MACHINERY. 35 



not stem ; the " root-stock," inasmuch as it does bear 

 scales and leaves, is truly a stem, even though it may be 

 beneath ground. The long creeping runners of " twitch " 

 (Triticum repens) are stems, so are the similar parts in 

 thistles and bear-bind (Convolvulus arvensis). The bulbs 

 of kohl rabi are clearly stems, for they bear leaves, or the 

 scars where leaves have once been. Beetroots, mangels, 

 radishes, turnips, parsnips partake of the nature of roots 

 and of stems ; that is to say, their lower tapering ex- 

 tremities are unquestionably roots ; their thick upper end, 

 surmounted by a crown of leaves, is as unquestionably 

 stem. There are anatomical differences such as the 

 presence of a root- cap, the absence of stomata in a root, 

 and differences in the mode of growth between roots and 

 stems, but they are not material to our present purpose. 

 It will be seen, from what has been above said, that the 

 definition of a stem (or of a branch, which is only a sub- 

 division of a stem), as that part of the plant told off to bear 

 leaves, admits of very wide differences of form. We have 

 already alluded to some of these differences, according as 

 the stem is above or below ground, covered with mere scale- 

 leaves, or bedecked like a timber tree with true leaves. The 

 pasture grasses and cereals have almost all hollow erect, 

 knotted stems ; the sedges, which resemble the grasses so 

 much, have mostly angular unjointed stems. The clovers 

 have a thick stock giving off branches which trail along the 

 ground. The hop coils round the supporting pole by means 

 of its climbing stem. Then there are the differences in dura- 

 tion associated with corresponding differences in texture 

 and internal construction. There are the so-called annual 

 stems, which would die down, even if they were not cut 

 down after one season's growth; there are the perennial 



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