28G MacMillan : ObservatiOxNS on Nereocvstis 



extension of the cell, while cross partitions are still being formed in 

 the cortical region outside of the sieve-tube area. I believe that 

 the fragmentation of the nucleus preventing the regular development 

 of cross partitions in the mother-cell of the sieve-tube is the occa- 

 sion of its failure to divide further and of its consequent passive 

 elongation. 



It was in this 12 cm, material that the best examples of cr)'p- 

 tostomata were discovered. I shall return to their discussion later 

 while considering the lamina. It suffices to mention at this point 

 that these peculiar piliferous organs of doubtful morphological 

 significance are present upon young stipe as well as upon young 

 lamina, but in mature plants I have not seen them. 



Origin of the P}icui}iatocyst. — A continuous ribbon of 3 mic. 

 sections was taken across the area of the pneumatocyst. The series 

 begins in definite stipe area, traverses the pneumatocyst and ends 

 in the base of the lamina. The plant measured 12 cm. in length. 

 In the undoubted stipe area the pith-web is seen to have its meshes 

 filled with gelatine and the first appearance of the pneumatocyst is 



I 1 



a small rift in the jelly near the center of the pith-web. As the 

 series passes over through the pneumatocyst this first rift is seen 

 to increase in size and others appear near it. At the same time a 

 distinct flattening of the stipe takes place and it is worth noting 

 that the primitive pneumatocyst is clearly elliptical in cross section 

 rather than spherical. As the series continues across the pneu- 

 matocyst area rifts in the jelly of the pith network become smaller. 

 The flattenin<:r of the whole area becomes more marked and in the 



t> 



■ 



base of the lamina the section has become five or six times as loner 

 as it is broad and shows the undoubted lamina characters. The 



h 



pneumatocyst, stipe and lamina are essentially the same in struc- 

 ture, but the bubble which forms in the jDith-web of thepneumato- 

 cyst area increases very rapidl}' in size and the vesicle thus formed 

 becomes finally the greater retort-shaped organ of the mature 

 plant. 



Stnictiu'c of older Stipes. — As the plant increases in 'size the 

 stipe continues to thicken by concentric walls which appear in 

 several of the sub-epidermal layers of the cortex. A plant 12 dm. 

 in length showed a stipe 5 mm. in diameter of which the central 

 cyHnder occupied 1.5 mm. In material of this age the cortical 



