246 BOTANY 



spiral inequalities of the hair. These two types tend to merge 

 into one another, but are differentiated by their behaviour in the 

 presence of moisture. If a single hair be placed under the micro- 

 scope without a cover-slip and be gently breathed upon, a number 

 of convolutions can be seen to run up and down the length of the 

 hair under the influence of increased humidity. The normal or 

 fixed convolutions, on the other hand, are altered but slightly by 

 this treatment. 



The development of the cotton hair takes place in a closed boll, 

 and on this account the space for its development is small compared 

 with the length to which the hair can grow. Consequently the 

 hair is compelled to double back upon itself many times with 

 the formation of a number of bends, and these bends are fixed in 

 the hair structure by the ensuing "secondary thickening" of the 

 wall. Suppressed convolutions are apparently due to spiral lines 

 of weakness. They have also been produced in hairs after 

 prolonged soaking in water followed by excessive twisting. 



For other points in connection with the microscopy of the 

 cotton hair the paper by Denham should be consulted, where the 

 presence of slip planes, formation of wall layers and other factors 

 are very fully discussed. 



THE STRUCTURE AND FORMATION OF LIGNIFIED 



TISSUES 



The early incidence of lignified tissues in the plant body is 

 intimately involved with the chemical processes taking place in 

 the meristematic region of shoot and root. The growing points, 

 i.e., the regions in which the formation of new tissues is taking 

 place, lie at the extreme ends of the branches of both shoot and 

 root. In every branch the basal region is the oldest and the apex 

 the youngest portion, and it is within the apical region that early 

 lignification occurs. 



In longitudinal section passing through the middle portion of 

 a growing point, as is well known, the end of the stem shows a 

 nearly flat or dome-shaped structure, and the leaves can be seen 

 to arise as slight projections on either side. In the apical region 

 of the shoot three regions can be distinguished — dermatogen, 



