246 Evans: Air chambers of Grimaldia fragrans 



In Fig. 7, which represents the apical region of another thallus, 

 a somewhat different condition is revealed. In this case the 

 first indication of an air chamber appears between the third and 

 fourth cells and is likewise in the form of a split, but this time the 

 split evidently began on the outside and extended inward. Al- 

 though the chamber is thus superficial in origin, there is no evidence 

 that a surface area has had its upward growth arrested, as Leit- 

 geb's explanation demands. The split clearly extends inward 

 from the original surface. The chamber between the fourth and 

 fifth cells is considerably deeper and broader, and it is clear that 

 its increase in size has involved further schizogenous processes. 

 The next chamber shown gives evidence of a further horizontal 

 extension. 



It would appear from these two figures that the air chambers 

 in Grimaldia fragrans owe their origin to a splitting of cell walls, 

 but that the place where the split first makes its appearance is not 

 always the same. It may be below the surface and extend out- 

 ward, in which case it agrees fully with the explanation advanced 

 by Barnes and Land; it may be at the surface and extend inward, 

 thus agreeing with Deutsch's account of Targionia hypophylla. 

 In the writer's opinion the figures published by Miss Hirsh and 

 Miss Black might be interpreted in the same way as Fig. 7, so 

 that there still seems to be no conclusive evidence that Leitgeb's 

 explanation ever applies. 



Figs. 8-10 yield further evidence as to the origin of the cham- 

 bers; they were all drawn from a single section, cut at right 

 angles to the long axis of the thallus, and show for the most 

 part superficial cells. In Fig. 8 the apical cell appears in the form 

 of a rectangle. Directly above it an air chamber reaching the 

 surface is shown between the fourth and fifth cells, corresponding 

 apparently with the chamber between the fourth and fifth cells 

 of Fig. 7. The schizogenous origin of this chamber seems clear, 

 but there is nothing to show whether the split began at or below 

 the surface. Between the third and fourth cells no signs of a 

 chamber can be discerned, although a superficial split may be 

 present like the one shown in FiG. 7. The figure at any rate gives 

 no evidence of a split beginning below the surface. 



FiGS. 9 and 10 are much more conclusive. They represent a 



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