374 



PHYSIOLOGY OF GROWTH. 



r 



The following' experiment is very instructive : A wire square 

 of about 60 mm. side, resting- on four wire pins, say 1 cm. high, is 

 dipped in the soap solution by means of a wire fixed at a suitable 

 place. The model, on removal from the solution, is placed in a 

 horizontal position, and on the soap sheet is laid anyhow a thread, 

 whose ends have been knotted together. If we pierce the sheet 

 with a glass rod at a point within the area bounded by the thread, 

 the sheet at once forms a minimal surface, and the thread conse- 

 quently disposes itself in a circle. 



To examine the growing points of roots, 

 we prepare median longitudinal sections 

 from the roots of Zea Mais or Hordeum. 

 Here also dermatogen, periblem and 

 plerome are present ; the root-cap, which 

 covers the tip of the root, is very striking. 



The cells of the growing points of stem 

 and root are seen to be in a state of active 

 division. The cells do not undergo vigorous 

 surface growth, leading to elongation of the 

 organs, until they are somewhat older. 4 



Regarding the further development of 

 the elements formed in the punctum vege- 

 tationis, it is to be emphasized that this 

 may take place either at the summit or at 

 the base of the newly formed organ. We 

 will here confine ourselves to what may be 

 observed in the growth of the shoot axes 

 of higher plants. 



In grasses but also in many other 

 plants the basal tissue of the internodes, 

 surrounded by the leaf sheaths, retains 

 for a considerable time a youthful character, 

 while the upper parts of the internodes 

 have already passed over into the state of 

 permanent tissue. This remarkable fact 

 of the existence of a basal intercalary 

 zone of growth can be readily demonstrated 

 by the following experiment : We cut an 

 internode frorn a haulrn of Secale, and 

 divide it into an upper and a lower half, 

 now place the two pieces with their lower ends in water, and 



FIG. 121. Lower part 

 of an internode from the 

 haulm of Secale. The 

 part a has been raised by 

 intercalary growth. 



