HIBERNATION 123 



CHAPTER X 



HIBERNATION ; THE STRUCTURE OF MODIFIED 



SHOOTS 



Having gained the foregoing knowledge as to the struc- 

 ture, functions, forms, and modes of growth of the vegeta- 

 tive organs of a few common plants, we will now pay 

 attention to some that are peculiarly modified. In the 

 preceding chapter we have been studying the formation 

 of dwarf shoots, rosettes, and scales all of which are 

 cases of reduction. Occasionally, however, the reverse 

 occurs, the roots, stems, or leaves becoming abnormally 

 enlarged, in which case they usually act as storage organs, 

 either of water, or organic food, or both. It is extremely 

 difficult, if not impossible, to say how these modifications 

 were brought about, but we can often suggest some useful 

 purpose they serve when once they are formed. 



Adverse conditions and their effect on growth. With the 

 changing seasons, plants are exposed to a great range of 

 conditions as to temperature, moisture, and light, which 

 greatly influence the power for work of the different 

 plant-organs. The parts most exposed to these changes 

 are the shoots above ground, the leaves being especially 

 sensitive. In temperate regions the winter conditions are 

 unfavourable for active root-absorption, and therefore for 

 active growth, as are the dry periods of many tropical and 

 sub-tropical countries. At the beginning of the adverse 

 period the first changes we notice are the withering and 

 shedding of the leaves of many shrubs and trees, and the 

 dying down of herbaceous shoots. The strong trunks and 

 branches of the former persist, enveloped in their coats of 

 cork, whilst their buds are protected by tough, brown 

 scales. But how do the more tender herbaceous plants 



