112 POPULAE EEKOBS. 



in a narrow space between the wood and bark, 

 where the cells are thin and soft and in the grow- 

 ing condition. In like manner, growth, wherever it 

 occurs, takes place only where there is soft tissue 

 of this character. For the security of the plant 

 against external injury it is important that there 

 should be as little such tissue exposed as possible, 

 hence, there are various means of protecting the 

 surface while in this condition, and of limiting 

 its duration when exposed. In ordinary forest 

 trees the usual period of expansion, during which 

 the leaves are produced and the season's growth is 

 formed, seldom exceeds two weeks, and this gener- 

 ally occurs in May or June, when there are few 

 insect or fungus enemies about and before the 

 weather has become hot and dry. An examination 

 of growing twigs at this time will show that they 

 elongate at first throughout their length, but that 

 soon the lower portion of the season's growth begins 

 to harden and ceases to lengthen. This fact may 

 be shown in any young shoot of considerable length 

 by noticing that the young leaves near the end are 

 close together, and gradually become farther apart, 

 as the internodes or spaces between them lengthen. 

 In roots the part which actually increases in 

 length is confined to a very short space just back 

 of the tip. It is thus protected, as has already 

 been shown, by the root cap at the extremity, and 



