304 



THE FORMS OF TISSUES 



[CH. 



from the curvature of the large surrounding sphere. But as the 

 latter continues to 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 order to epitomise the foregoing facts let the annexed 

 diagrams (Fig. 113) represent a system of three films, of which 

 one is a partition- wall between the other two ; and let the tensions 

 at the three surfaces, or the tractions exercised upon a point at 

 their meeting-place, be proportional to T, T' and t. Let a, j3, y 

 be, as in the figure, the opposite angles. Then : 



(1) If T be equal to T', and t be relatively insignificant, 

 the angles a, j8 will be of 90°. 



Fig. 113. 



(2) If T = T', but be a little greater than t, then t will exert 

 an appreciable traction, and a, j8 will be more than 90°, say, for 

 instance, 100°. 



(3) If T- T' = Athena, ,^,y will all equal 120°. 



The more complicated cases, when t, T and T' are all unequal, 

 are already sufficiently explained. 



The biological facts which the foregoing considerations go a 

 long way to explain and account for have been the subject of much 

 argument and discussion, especially on the part of the botanists. 

 Let me recapitulate, in a very few words, the history of this long 

 discussion. 



Some fifty years ago, Hofmeister laid it down as a general law 

 that " The partition- wall stands always perpendicular to what was 

 previously the principal direction of growth in the cell," — or, in 

 other words, perpendicular to the long axis of the cell*. Ten 



* Hofmeister, Pringsheini's Jahrb. in, p. 272, 1863; Hdb. d. 

 1867, p. 129. 



Bot. I, 



