480 



THE FORMS OF TISSUES 



[CH. 



grow its curvature decreases, and so likewise does the inward 

 pressure of its surface ; and accordingly the little convex partition 

 bulges out more and more. 



In the ordinary meristematic tissue of a plant, the new partition- 

 wall within a dividing cell will generally meet the old walls at right 

 angles to begin with, because its tension is usually small compared 

 to what theirs has become. But as the system grows and the old 

 wall strengthens, the tensions of all three walls become approxi- 

 mately the same; and they tend towards a new position of equi- 

 librium, in which (as seen in optical section) they meet as before, 

 at co-equal angles of 120°*. 



A B 



Fig. 169. Cambium cells after division, altering from A to B. 



The biological facts which the foregoing considerations go far to 

 explain and account for have been the subject of much argument 

 and discussion on the part of the botanists. Let me recapitulate, 

 in a very few words, the history of this long discussion. 



Some seventy years ago, Hofmeister laid it down as a general, 

 but purely empirical, law that "The partition- wall stands always 

 perpendicular to what was previously the principal direction of 

 growth in the cell" — or, in most cases, perpendicular to the long 

 axis of the cellf. This contains an important truth; for it is as 

 much as to say that the cell tends to be divided by the smallest 



* J. H. Priestley, Studies. . .of cambium activity,. New Phytologist, xxix, p. 101. 

 1930. Cf. also J. J. Beijer, Vermehrung der radialen Reihen in Cambium, Rec. 

 de trav. hot. Need, xxiv, pp. 631-786, 1927. 



Hofmeister, Pringsheini's Jahrh. iii, p. 272, 1863; Hdb. d. physiol. Bot. i, 

 p. 129, 1867; etc. Hofmeister adds the somewhat curious qualification: "Wohl- 

 bemerkt, nicht senkrecht zum grossten Durchmesser der Zelle, der mit der Richtung 

 des starksten Wachstums nicht zusammenfallen braucht, und in sehr viel Fallen 

 in der That auch nicht mit ihr zusammenfallt." 



