1894.] Comparative Histology of Pulvini. » 483 
petiolule is transformed into a motor organ, this I term the 
secondary organ. (Contrast with Penhallow’s paper.) In 
the case of bi-pinnate leaves, the petiolules of the second- 
ary leaflets may be transformed into motor organs and these 
Iterm ¢ertzary organs. Theterm ventral has been used to 
indicate the upper side of the organ, leaf-blade or petiole, 
that is, the side toward the axis in the bud, the dorsal being 
the lower side or that which is away from the axis in the bud. 
Following the example of Penhallow!®, I have used the term 
hypodermal tissue to mean the tissue between the stele and 
the epidermis which is derived from the periblem. Vines** 
calls this extra-ste/ar fundamental tissue. Inthe motor organ 
the pith is either pushed to one side and the cell-walls be- 
come thickened or it remains in the center of the stele; when 
the former is the case, the stele is generally somewhat kidney 
shaped in cross section, the remnant of pith occupying the de- 
pression. This depression I have termed the Ac/um of the stele 
in order to facilitate description. In the descriptions I have 
given the external appearance of the motor organs and the 
changes in position of leaves from normal diurnal positions 
to the nocturnal positions. Ihave then in each case given 
the anatomy of the organs, beginning with the primary and 
following this with the secondary and the tertiary organs 
when present. 
From the twenty-five species described I select one, which 
though treated at greater length than the majority Is best 
adapted for illustration. 
Melilotus alba. 
Melilotus alba shows well developed motor organs and con- 
_ Sequently very marked movements. It has a primary motor 
gan which will be noticed as a slight thickening of the 
Petiole just above the bract-like stipules, each of which con- 
tains a well developed schizostele. These schizosteles arise 
alf way around the central stele of the axis and pass up- 
_ Wards and towards the base of the pulvinus, where just be- 
: fore uniting with the middle schizostele, they are ge ett 
pent angles to the axis. These three schizosteles of whic 
_ the center one is the largest unite in the center as they pass 
Bough the motor organ proper. Besides being an enlarge- 
|, PIOPE SOT te ee eee 
1, Penhallow: 
"Vines; deAe ¥en wick of Botany 146. 1894. 
- 
