496 PHYSIOLOGY. 



from the rods. The cellulose coat of the cell is secreted by the ectoplasm. 

 The molecular structure of protoplasm is probably varied in different 

 cases. The investigation of this obscure subject is of immense import- 

 ance, as the functions of life seem to depend on the activity and behaviour 

 of the molecules of protoplasm. They have been recently called plasti- 

 dules, and, unlike the molecules of the cell-coat, are not crystalline. 



The protoplasm is not always readily discoverable in living cells, on 

 account of its close apposition to the cell-wall, but it ma}^ be detected by 

 the application of a weak solution of iodine, which colours it brown, and 

 soon causes it to contract and separate from the cell-membrane (fig. 546). 

 The contraction is disadvantageous in some cases, if it go very far, as the 

 layer becomes applied upon the inner cell- contents. The structure is 

 very well seen by placing portions of the green tissue of leaves, &c. 

 (which retain the primordial utricle after acquiring their full size), of the 

 pulp of fruits, the leaves of Mosses or Liverworts (fig. 546), or the fila- 

 ments of Confervoids, in alcohol, or treating them with dilute nitric or 

 hydrochloric acid. The primordial utricle then separates from the cell- 

 wall without becoming much discoloured. The chemical properties of 

 protoplasm indicate its affinity to albuminoid substances. It is, moreover, 

 mixed with oily substances and inorganic or incombustible materials. 



Vacuoles, &c. In young cells, such as those in the cambium- 

 layer of stems, in the growing parts of leaves, &c., the protoplasm 

 nearly fills up the cavity, or at all events occupies all the space not 

 filled by the nucleus. By degrees, as the cell expands, spaces called 

 vacuoles make their appearance in the protoplasm, filled with 

 watery cell-sap (fig. 547) ; they may be regarded as drops of 

 watery fluid surrounded by protoplasm, which latter is thus trans- 

 formed into a kind of froth, which is often finally displaced so 

 entirely by the cell-sap that it forms merely a layer applied against 

 the primordial utricle. In many zoospores these vacuoles are seen 

 to contract and expand, probably from the varying contractility or 

 degree of turgescence of the surrounding protoplasm. These 

 movements are identical with similar phenomena witnessed in Pro- 

 tozoa, and are the first indications of a respiratory or circulatory 

 system. 



Movements in the protoplasm, rendered evident by the movement of 

 the granules floating in it, occur in many plants, probably in all, and are 

 attributed by some to contractility of the protoplasm, by others to alter- 

 nate turgescence and emptying of certain portions of the protoplasm. 

 They are strictly analogous to the movements seen in Amoeba and other 

 similar low animal organisms. The protoplasm is said to be chambered 

 when the cell-sap is traversed by several anastomosing plates of protoplasm. 



Nucleus. In the protoplasm of most young cells, and persistent 

 through life in the parenchymatous structure of some plants, as of 

 the Orchidacese, occurs the globular or lenticular body called the 

 nucleus of the cell, or cytoblast (figs. 547, n, & 548), discovered by 

 Bauer, and first investigated by Kobert Brown. This appears to 

 be a mass of substance identical with protoplasm, and it mostly 



