222 PROCEEDINGS OF THE ACADEMY OF [1899. 



Branch 3. — 

 1 miu.— Respouse to thermal action. 



3 niiu, — Nearly fully expanded. 

 5 min. — Fully expanded. 



Branch Jf. — 



1 min. — Response to heat stimulus, 



2 min. —Snow which had been enclosed by the inrolled leaf 



dropped out. In fig. 2, the leaves here are yet more 

 fully expanded and the petiole has turned slightly up- 

 ward, as a critical comparison of figs. 1 and 2 will de- 

 monstrate. 



4 min. — Leaves nearly fully expanded. 



5 min. — Leaves fairly expanded, but the petiole has not re- 



sponded so quickly, as in the other experimental 

 branches. 



Fig. 3 illustrates a branch after it has assumed its normal dorsi- 

 veutral position. 



A cross-section of the leaf shows the following arrangement of 

 cells. There is a thick cuticle developed on the upper surface. 

 Beneath this there are two rows of epidermal cells ; the upper row 

 has a thick external wall of cellulose. Beneath the epidermis, 

 there are several well-defined layers of palisade cells, and then 

 follow the loose parenchyma cells, more or less compacted together, 

 succeeded by the lower epidei-mis provided with stomata and multi- 

 cellular hairs, so disposed that they form a flat surface of inter- 

 locked branches as a protection against too rapid loss of internal 

 water. A striking feature of most of the cells, especially in the 

 upper epidermis, palisade tissue and loose parenchyma tissue, is 

 the intercellular communications, which are visible under ordinary 

 treatment and powers as depressions in the cell-wall. These are of 

 importance as part of the mechanism of movement. 



Research has shown that the movement in the leaves of Mimom 

 pudica L., M. sensitiva L. , Oxalis bupleurifolla A. St. Hil. , and 

 other sensitives, is brought about by the extrusion of water from 

 the pulvini into the contiguous stems and petioles, resulting in the 

 contraction of the pulvini. When the absorbing tissue of the 

 pulvini have again taken up water, and become tense and firm, 

 they will react again to new stimuli. The study of the cell struc- 

 ture of the leaf of Rhododeudro)i maxiniimi L. leads the writer to 



