68 Introduction to Botany. 



horse-chestnut 'and of hickory are hard and dry and well 

 adapted to give protection against mechanical injury, 

 while the hairs growing over the scales tend to keep them 

 separated slightly, and so form dead air spaces which retard 

 the escape of water from the succulent parts of the bud, 

 and, on account of the slight conductivity of air for heat, 

 protect the inner parts against sudden changes in tempera- 

 ture. In the winter buds of the cottonwood, the tender 

 parts are prevented from drying up by resinous substances 

 which hold the scales closely together. The amount of 

 protection required by buds varies greatly for different 

 plants, and depends largely upon the ability of their pro- 

 toplasts to withstand adverse conditions. The buds of 

 some plants are able to pass through severe winters in 

 safety without any special means of protection. Even in 

 the arctic regions there are plants whose buds are able to 

 survive the intense cold while in a succulent, half -formed 

 condition. The degree of protection given to buds seems, 

 therefore, to be regulated by the needs of the living cells. 



50. Disposition of Leaves in Buds. The manner in 

 which the embryonic foliage leaves are packed away in the 

 bud is not the same in all cases ; but, however these details 

 may vary, the leaves already possess in the bud the regular 

 angular divergence on the stem which is found in mature 

 shoots, and this facilitates their disposition in the very small 

 compass of the bud. 



51. Unfolding of Winter Buds. The unfolding of 

 winter buds in the spring is essentially the resumption of 

 growth in parts already formed. The short internodes and 

 the minute leaves quickly elongate and expand, and almost 

 in a day the trees are again covered with foliage. Such 

 remarkable development would be impossible if reserve 

 food materials were not already at 'hand for the building 



