THE SENSITIVE-PLANT. 



545 



motile organ consists of a. thick covering of parencRyma enveloped by" a feebly 

 developed epidermis devoid of stomata, and which surrounds an axial, flexible, 

 but only slightly extensible strand of vascular bundles ; the individual bundles of this 

 strand arise from the vascular bundles of the shoot-axis, and at the other end of the 

 organ where they enter into the leaf-stalk they become again isolated, as has been 

 described in the case of the Bean. The parenchyma consists of rounded cells which 

 in the neighbourhood of the vascular strand enclose rather large intercellular spaces 

 containing air; these spaces are much smaller, however, in the 10-20 outer layers 

 of the parenchyma, and are altogether absent in the neighbourhood of the epidermis. 

 These air-cavities between the cells communicate with one another from the strand 

 to the middle layers of tissue ; the very small intercellular spaces of the outer layers 

 of cells, on the contrary, appear as separate three-cornered lacunae, and, in specimens 

 under the microscope, are filled with water. The cells of the lower side of the organ 

 are thin-walled, those of the upper side have walls about three times as thick, which like 

 the former are traversed by numerous pore-canals. In addition to abundance of 



Hir.. 373.~Leaf of j1/i»n;j«/«(i?;Vrrt, half natural size. A the diurnal non-irritated condition ; 7? tlie nocturnal 

 position, or after irritation by a shocic (after Duchartre). 



protoplasm, nucleus, and small chlorophyll-corpuscles and starch-grains, the cells 

 contain each one large spherical drop suspended in the sap-cavity, and which consists 

 of a solution of tannin surrounded by a delicate pellicle. Unger also found similai" 

 structures in the motile organs of Desmodium gyrans and Glycyrrhiza, which, how- 

 ever, are not sensitive to contact and vibration. Again, the organs of Mimosa are 

 irritable even in the young state, when the cell-walls in the upper half of the sheath of 

 parenchyma are not yet thicker than those of the lower, and the spheres referred 

 to are still wanting. Too great importance therefore is not to be attributed to these 

 anatomical matters, in respect to the specific phenomena of irritability ; and this so 

 much the less, because the corresponding anatomical characters of the irritable tissue 

 in other organs to be considered later, diflFer in several respects, although the 

 phenomena of irritability are the same in all essential points. 



When a Sensitive-plant is left to itself during the day, its petioles are 

 directed obliquely upwards, the secondary petioles with their leaflets being-extended 

 almost exactly in one plane, as in Fig. 373^. Any vibration (unless very gentle) affects- 



