1894. ] Comparative Histology of Pulvini. 485 
directly without any twisting, while at other times a slight 
twist is made. They thus assume nearly the same position 
as the leaflets of T. repens, only the position seems different 
on account of the upright growth of ‘the axis. 
Darwin’? describes the ‘sleep movements” of a large num- 
ber of species of the genus Melilotus, and as the type of their 
movements, he selects a case altogether different, in that the 
movement is the exact reverse of what I have found in Melil- 
otus alba, although this is one of the species that he enum- 
erates. Special pains was taken to make sure of this move- 
ment and plants were visited again and again with always the 
same result. It could hardly be that Darwin made a mistake 
and as he mentionsa similar movement in one species at a differ- 
ent time, it is possible that the movement may sometimes 
differ in the same species. From this reversion of the move- 
ment to the Trifolium type, Darwin is inclined to speculate, 
_. and regards it as an indication of the close relation of the 
two genera. Since all photeolic movements are either up or 
down I can see no basis for Such’ speculation. 
Primary organ.—The pulvinus of the petiole as seen in the 
Cross section is kidney shaped, fig. 4, the hilum being di- 
rected upwards or towards the ventral side of the petiole. The 
dorsiventral diameter is about 1.4", the lateral diameter 
1.75", while the length is only about 1.4". From these 
Measurements it will be seen that the organ is quite short be- 
ing only as long as its dorsiventral diameter. In the diurnal 
Position the organ has a single transverse depression on each 
Side, that on the ventral side being somewhat closed up, but 
not as deep as the one upon the dorsal side. In the noc- 
turnal position there is a marked change in the outline; the 
transverse depression.on the dorsal side becomes less marked 
as the petiole approaches the axis, while the motor organ, on 
its ventral side, is thrown into transverse folds for its entire 
length, (3 or 4 folds) the depth of the depressions depending 
Upon the degree of movement that has taken place. gt 
seneral rule the depressions are deeper at the base of the 
°rgan and become more shallow towards the distal end. 
here is a gradual change in the epidermis in passing from 
the leaf or petiole, so gradual indeed that it is impossible to 
tell €xactly where the cells belong to the one or the hae 
region. The epidermal cells of the pulvinus are of abou 
*? Darwin: Movement of Plants. 346, jig. 749. 1880. * , 
