678 SUMMARY OF CURRENT RESEARCHES RELATING TO 



occurrence. Observations now and lately recorded give us the power 

 of a " clearer insight into such jihenomena as the downward move- 

 ment of a sensitive leaf upon stimulation, of the wonderful action of a 

 germinating embryo on the endosperm cells, even on those which are 

 most remote from it, of the action of a tendril towards its support," and 

 of various other phenomena in connection with general cell mechanism. 

 The author describes the protoijlasmic continuations between cells 

 in various tissues, treating especially of pulvini and endosperm cells. 



Pores in the Outer Walls of Epidermal Cells.* — It has long been 

 known that the outer walls of the cells of the epidermis are in some 

 cases perforated ; and Mettenius has described the same phenomenon 

 in the outer cell-walls of the very thin leaves of certain Hymeno- 

 phyllaceaa ; the cell-walls being in these cases also remarkably curved 

 and folded. H. Ambronn has made a careful examination of all these 

 cases, and divides them into two classes : — those in which the perfora- 

 tions perform the same function as the pores in the cell-walls of 

 internal tissues, viz. the interchange of fluids or of gases from cell to 

 cell ; and those rarer ones in which they subserve some other function. 



The object of the thickenings and foldings which occur in the cell- 

 walls of the Hymenophyllaceae, and also in the walls of the e2)idermal 

 cells of the leaves of Couifei.Te, is api:)arently to increase their power 

 of resistance to traction in the tangential direction. The perforations 

 under consideration are always found in connection with these 

 thickenings and foldings, and Ambronn gives a detailed mechanical 

 explanation of his reasons for adopting the view that these cavities — 

 which are really fissures, but have from the outside the appearance 

 of dots — are the result of the unequal growth to which these thicken- 

 ings are due. 



Perforations of this kind are very common in the epidermal cells 

 of grasses, both in the stem, leaves, and leaf-sheaths; also in many 

 Juncacea3 and Cyperacese. Among vascular cryptogams they occur, 

 not only in the Hyinenophyllacea), but also in Equisetum. In Coniferse 

 they are found especially in the genus Abies. They are met with also 

 in the peculiar epidermal cells of the leaves oi Amaryllis formosissima. 

 In dicotyledons they are also not uncommon. 



Another class of perforations is connected with the formation of re- 

 ticulate thickenings, wliich continually increase in thickness so that the 

 meshes between them become constantly narrower until at length they 

 have the apj carauce of pore-fissures. A good instance of this occurs 

 in the genus Cycas, the only genus of Cycadere which possesses these 

 reticulate thickenings. They are found also in Coniferae, as in the 

 leaves of several species of Pinus and in Cedrus Deodara. 



A peculiar kind of thickening, associated with pores, is found in 

 the outer walls of the epidermal cells of the leaves of some species of 

 Epacrideae, especially in the genera Epacris and Leucopogon. 



Physiology of the Undulations of the Lateral Walls of the 

 Epideriuis.'l' — J. Vesquc finds that the epidermis of leaves has very 



* Pring.-heim's Jahrb. "Wiss. Bot, xiv. (18S3) pp. 82-111 (1 i.l.)- 

 t Comptcs Koudus, xcvii. (1883) pp. 201-3. 



