184 The Anatomy of Plants BOOKII 



linked together by chemical affinity into a reticular frame- 

 work, in the meshes of which the water is retained by 

 capillarity or surface tension. Additions to the substance 

 of such cell walls are made by the apposition of new 

 layers to their external surfaces, and the lamellar appear- 

 ance, known as stratification, is due to such deposition. 



Strasburger supported the last part of his hypothesis by 

 actual observation of the deposition of microsomata from 

 the protoplasm in sheets upon the existing walls. The 

 so-called striation he explained by the deposition of the 

 granules in each layer in oblique lines down the longer 

 axis of the cell. He showed further that the lines of such 

 striation are confined to each layer of deposition, though 

 successive layers may be superposed. The appearance of 

 cross striation so often seen is due to the absence of such 

 superposition. Strasburger thus vindicated the view of 

 Wigand in 1856, that each layer deposited has its own 

 striation marks. 



Strasburger's explanation of the growth of surface on 

 this hypothesis was the greater stretching power of a 

 wall after a new lamina has been deposited upon its 

 surface. 



For a time, under the inspiration of Strasburger's name, 

 the theory of growth by apposition superseded the older 

 theory of intussusception. In 1886, however, another hypo- 

 thesis was brought forward, which met with considerable 

 acceptance. It was due to Wiesner, and was based upon 

 a conception that had originally been advanced by Hof- 

 meister in 1867, that the cell wall is a living structure, 

 though in its details it was altogether new. Wiesner sug- 

 gested that while young and growing the membrane is 

 made up of an aggregation of granules arranged in rows, 

 and surrounded by protoplasm, the granules being con- 

 nected together by protoplasmic threads. These granules 

 he named dermatosomes. On this conception the wall 



