Growth. 



II 



amount of growth o£ this part of the stem will be denoted by 

 the distance of the lowest mark from the apex of the stem. 

 Compare the lengths of the different intervals. 

 One of these, about the third or fourth from _the 

 apex, will be found to have made the greatest 

 elongation, and may be designated as t he zon e 

 of inaxin iuin_ gromi th. Continue the measure- 

 ments three days more, and note that the zone 

 of maximum growth has changed its position. 



It was doubtless seen during the previous 

 measurements that the stem is made up of a 

 number of sections or i ntern odes ; the region in 

 which the sections are joined constitutes the 

 Hg^gS, at which p oints the leaves arise , being 

 borne on the ejidofjnteriiodes. Examine the 

 record of measurements made above, and note 

 whether all parts of the older internodes elongate 

 with the same rapidity. If possible, repeat both 

 observations with the branches of trees. 



2. Location and arrangement of growing tis- 

 sues. — Cut a thin cross-section of the stem used in the previ- 

 ous experiment, by means of a sharp razor and a suitable 

 clamp. Examine with magnifications of about sixty, and four 

 hundred. In s ections tak en from the zone of maximu m growth , 

 it will be found that the greater part of the stem is composed 

 of s mall thin-w alled cells. About half way between the center 

 of the section and its outer edge may be seen groups of cells 

 with heavier and variouslj^ differentiated walls, constituting the 

 fibrovascular bundles, which are arranged in a cylindrical shell 

 in the stem (Fig. 3, A). Cut a section from the same stem, 

 a few centimeters lower down, and the bundles will have 

 formed a ci rcular band which will exhibit two distinct regions . 

 A portion lying toward the center of the stem constitutes 

 the xylemj and consists mostly of vessels and tracheids with 



A 



Figf. 2.— De- 

 vice for mark* 

 ing stems with 

 India inlc A^ 

 thread; B, 

 wire. 



