192 Prof. F. Cohn on the Contractile Tissue of Plants. 



elasticity of the cell-tissue, or the property to which a structure 

 owes its permanence of form and its capability of renewing that 

 form after disturbance from any cause. Moreover, the elastic 

 powers of the stamens differ, as in muscle, under different cir- 

 cumstances. In the irritable stage the elasticity is great, but the 

 extensibility small ; and on the contrary, when the irritability is 

 lost, the elasticity is decreased, and the filaments can then be 

 readily extended. Still the elasticity remains so far as to assert 

 its power by shortening the filaments when the extending force 

 is removed ; and this holds true even after their vitality has 

 ceased. 



18. We may probably arrive at a better apprehension of the 

 phenomena detailed by endeavouring to discover in what tissues 

 the contracting and the extending forces of the irritable stamens 

 reside. The filaments of Centaurea are composed of very deli- 

 cate cells, mostly somewhat longer than broad. Their softness 

 or delicacy is so great that they are easily crushed by the glass 

 cover in a microscopical examination. They are cellulose in 

 chemical composition, and covered by an epidermis consisting of 

 still larger though very delicate cells, three to four times as long 

 as broad. Their outline is gently undulating, and their proto- 

 plasm is thick and coarsely granular ; externally they give off 

 conical-cylindrical hairs from over the septa between adjoining 

 cells, so that these hairs are themselves divided by the longitu- 

 dinal septa, being, as it were, prolonged into them. A cuticle 

 encloses both the epidermic or epithelial cells and the hairs 

 growing from them. In the interior of each filament is a bun- 

 dle of spiral vessels with prosenchyma-cells and air-passages. 



The question is, whether the cellular structure possesses, as a 

 whole, the extending and contractile power, or whether the several 

 tissues distinguishable have different and special functions. 

 Microscopic examination can afford no positive answer to this 

 question ; but the following deductions may be made : — 



a. The contraction proceeds in the cell-structure at large. If 

 not, contraction would involve folds or wrinkles at parts ; and 

 such are not discoverable, b. On the other hand, the vascular 

 bundle in the centre exhibits no activity in the process of con- 

 traction; for in a contracted filament the vessels are not in a 

 state of tension, but wavy. c. The stretching of the different 

 parts varies in degree greatly ; for when a filament is slit longi- 

 tudinally, it curves itself spirally, and so that the cut surface 

 occupies the convex side. This shows that the tissues nearest 

 the epidermis undergo greater shortening. Morren has made 

 the remark that the centre of motile stamens possesses contrac- 

 tile power, and that the superficial epidermis and cuticle consti- 

 tute the elastic portion ; but Cohn inclines to the opinion that 



