3°6 
Kisch . — The Physiological Anatomy of 
be seen that the delicate mesh cells, a, are formed in the ordinary radial files 
by a certain number of the cells being left thin-walled, and becoming 
extended in the tangential direction, whilst typical periderm cells, b , are 
present both within and without. 
By examining the meshes in tangential section it is found that these 
thin-walled cells divide by horizontal septa into chambered cells, while each 
segment may undergo further vertical divisions (Text-fig. 19, A), so that 
later the outline of the original extended prosenchymatous cell is lost ; but 
Text-fig. 19. Sigillaria spinulosa. Tangential sections through two meshes, A and B. 
A still shows the extended thin-walled prosenchymatous cells of the mesh. In B the prosen- 
chymatous form has been lost as a result of further divisions, but the pointed end-segments can still 
be seen at e . Will. Coll., 668. x 100. 
the occurrence of the pointed end-segments still gives the clue to the origin 
of the mesh (Text-fig. 19, B, r). 
As there are no ^thin-walled meshes left in the inner portion of the 
tissue, and as they increase in size towards the exterior, it would seem 
probable that the smallest meshes were the first formed, and that larger 
groups of cells were left thin-walled as time went on, i. e. that the phellogen 
was near the outer margin. 
The Dictyoxylon arrangement perhaps presented a means of economy 
of thickening material and adaptation to a continually increasing circum- 
