Notes. 
369 
ON THE OCCLUSION OF THE STOMATA IN TRADESCANTIA 
PULCHELLA. — Whilst studying the effects of traumatic stimuli on the leaves of 
Tradescaniia pulchella an interesting instance of stomatal occlusion was discovered. 
The wounds were made by incisions and by the removal of portions of the lamina, 
and it was thought at first that the occlusion was in the nature of a wound response , 1 
tending to check excessive transpiration. The examination of a large number of 
mature uninjured leaves showed, however, that the stomatal occlusion was a constant 
feature. 
In transverse section the leaves show an epidermis composed of very large cells, 
devoid of chlorophyll, those of the under side, to which the stomata are confined, 
■ d 
Fig. (a) Portion of leaf of T. pulchella in transverse section to show distribution of tissues 
and occluding growths; ( 6 ) and (c) show other views of occluded stomata, u.e. upper epidermis ; 
l.e. lower epidermis; me. mesophyll; g.c. guard cells of stoma; a.g.c. auxiliary cells ; oc. occluding 
growths. 
being somewhat the smaller. The epidermis encloses, in the young laminae, a perfectly 
homogeneous mesophyll composed of rounded cells with abundant chlorophyll. In 
the mature leaves the layer of mesophyll immediately abutting on the lower epidermis 
enlarges somewhat and seems to lose some of its chloroplasts. It is from this layer 
that the occluding growths arise, as bladder-like swellings which gradually fill up the 
cavity below the guard cells in a similar manner to tyloses (Fig., a, b, c). When 
1 Occlusion by parenchymatous ingrowths has been recorded for Equisetum , as a traumatic 
response, by Strasburger (Leitungsbahnen, 1891 ; Der Bau der Kryptogamen), and I have also 
observed it in the leaves of Cycas revoluta. 
