Growth 313 



The rate of growth extension in seedlings or other small 

 plants may be conveniently recorded by means of auxa- 

 nometers of various forms. A desirable type of this in- 

 strument produces a record by the following principle : 

 A cord attached to the growing organ passes over a small 

 wheel to a weight which takes up the slack induced by 

 growth. The rotation of the small wheel carries with it a 

 larger wheel from \vhich a cord is connected with a pointer 

 working against carbonized paper on a revolving drum. 

 In this manner a continuous record is secured of the incre- 

 ments of growth. 



181. The formation and exfoliation of leaves. - - Origi- 

 nating in the developing periblem just behind the apex the 

 young leaves arise, in spiral order, or verticillately disposed 

 about the stem. These are at first small protuberances 

 resulting from cell divisions parallel to the surface (peri- 

 clinal divisions) ; but in time they flatten and grow faster 

 than the stem apex. They curve over the stem apex 

 more or less to form a bud. 



In many annuals, and in perennial herbs, but only 

 occasionally in trees, the leafy axis continues to elongate 

 throughout the so-called growing season. If the shoot 

 apex thus constantly elongates, each leaf in succession 

 remains a part of the bud for only a short space of time, 

 since by further growth, - - more rapid on the upper sur- 

 face, - - complete exfoliation is soon effected. The exact 

 point of origin of the leaf back of the stem apex depends 

 in general upon the type of leaf arrangement in the 

 species of plant. In the axil of each young leaf there 

 is commonly formed later a region of growth destined to 

 become a lateral bud. This bud also originates ordinarily 



