20 METABOLISM 



dilute solutions of free acids and of alkalis may be observed to take place 

 without any injury ensuing to the protoplasm. 



The penetration of protoplasm by aniline dyes, a knowledge of which we 

 owe to Pfeffer (1886), is a phenomenon of great interest and importance. 

 Since colouring matters naturally dissolved in the cell-sap are unable to undergo 

 exosmosis, so long as the protoplasm is in a normal condition, the 

 protoplasm has been generally regarded as impermeable to dyes ; at the 

 same time, we have long been aware that dead protoplasm can absorb and 

 accumulate many pigments. The majority of the aniline dyes are poisonous 

 to the cell, and unless very dilute solutions be employed, they kill the proto- 

 plasm, before they penetrate it. Among the relatively non-poisonous anihne 

 dyes, methylene blue ranks first, for the plant can endure a solution of i in 

 100,000, or even i in 10,000, without suffering any injury. A solution of 

 methylene blue of a strength of i in 100,000 exhibits a beautiful blue colour 

 when in a layer several centimetres thick, but the colour is scarcely noticeable 

 in a glass tube i mm. in diameter, and cannot be distinguished at all, even with 

 the aid of a microscope, in a capillary tube o-imm. broad. If the cell-sap of 

 Spirogyra consisted of such a solution it would show no coloration, so that 

 a colouring body may very easily penetrate the protoplasm without making 

 itself apparent in the vacuole. Pfeffer, however, found that the roothairs 

 of Trianea after a short time exhibited an obvious blue coloration when treated 

 with a solution of methylene blue of i in 100,000, whilst in Spirogyra blue 

 granular masses made their appearance in the cell-sap. Diffusion of the dye 

 through the protoplasm must therefore not only have occurred, but a subsequent 

 accumulation of the dye in the vacuole must also have taken place. This accu- 

 mulation results from the transformation of the substance, after entry, into a 

 form which cannot again pass out, thereby making room for the further entry of 

 the pigment. This phenomenon is of the greatest significance in relation to the 

 absorption of materials by the plant ; for diffusion may result in the accumu- 

 lation in the cell-sap of materials in request, until the degree of concentration 

 inside is equal to that outside. Since, in most cases, the solutions available 

 for the plant in nature are extremely dilute, only very small amounts of 

 material can enter it by diffusion only, but if these substances so entering can 

 be stored away in an insoluble, or, at all events, in another and non-diffusible 

 form, continued entry of the material in question is possible. 



The cell shows, therefore, certain characteristics which are of fundamental 

 import in the well-being and hfe of the plant ; one of these is its capacity for 

 absorbing materials, not indiscriminately just as they are presented to it, but 

 selectively, both from a qualitative and quantitative point of view. Hence 

 a substance widely diffused in nature may be altogether wanting in the cell, 

 simply because it is not capable of diosmosis, whilst a comparatively rare sub- 

 stance may be accumulated in considerable quantity. It is only in a few cases 

 that we are exactly acquainted with the cause of the storage, e.g. that of methy- 

 lene blue by the root-cells of Lemna. In this case the dye is united with tannin, 

 and tannate of methylene blue is incapable of penetrating the protoplasm in 

 any direction. No absorption or storage of this body occurs if the cell be placed in 

 tannate of methylene blue instead of methylene blue itself, nor does any exos- 

 mosis of the tannate formed in the cell occur if the cell be again placed in water. 

 If, however, a drop of citric acid be added to the water, after a short time the 

 blue colour gradually disappears inasmuch as a process the converse of storage 

 takes place. First of all a very little citric acid enters the cell and unites with 

 the methylene blue ; as a result of this union, room is created for the entry 

 of more citric acid, and as citrate of methylene blue is capable of penetrating 

 the protoplasm, all the blue colour in the cell in the long run undergoes 

 exosmosis. Storage of such compounds does not always take place in such a 

 simple manner as that just described ; sometimes the changes are less, some- 



