372 



PHYSIOLOGY OF GROWTH. 



factors (temperature, light), I refer the student to the treatise by 

 Gr. Kraus in the Botanisclie Zeitung, and to my Text-Book of Plant 

 Physiology. In these places also reference is made to the im- 

 portant, though certainly not yet sufficiently studied phenomena 

 of periodicity in tissue tension. 



151. The Growing Points and Growth in Length. 



The growing points of plant organs are of very different 

 character as regards detail. For our purpose it is sufficient to 

 examine accurately the growing point of Hippuris vulgaris. The 

 construction is analogous in other cases. We cut off the terminal 

 buds of very strong shoots to a length of about 1 cm., remove 

 the leaves as well as possible, and prepare delicate longitudinal 



sections from the buds. In 

 Fig. 120 is depicted the 

 appearance of a very suc- 

 cessful median longitudinal 

 section. In order to render 

 the arrangement of the 

 cells of the vegetative cone 

 clear and distinct, it is 

 necessary to clear the sec- 

 tions. This is done by 

 treating them with con- 

 centrated potash solution, 

 and then after washing 

 with water laying them 

 in concentrated Acetic acid. 



FIG. 120. Longitudinal section through the 

 vegetative cone of Hippuris vulgaris. d, derma- The dividing walls of the 



f 



togen ; pr, periblem ; pi, plerome ; /, leaf primor- 

 dium. Magn.240. (After Strasburger.) 



11 i nvpT , w Vn'p"h 

 layers, Wffi 



placed over one another 

 like mantles, form a series of confocal parabolas. The outer- 

 most layer of cells, from which the epidermis proceeds, we 

 term the dermatogen, d. Then follows the periblem, pr, made 

 up of several layers, and yielding the cortex of the stem. Finally 

 comes the plerome, pi, from which, as can be made out further 

 down in the section, is derived the axile vascular bundle cylinder 

 of the stem. In Hippuris, therefore, but not by any means in 



