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SCIENCE. 



[N. S. Vol. XII. No. 300. 



the cell, but there is no evidence that he 

 regarded it as living. It was not until 

 the renascence of research in the forties, 

 to which I have already alluded, that 

 any real progress in this direction was 

 made. The cell-contents were especially 

 studied by Naegeli and by Mohl, both of 

 whom recognized the existence of a viscous 

 substance lining the wall of all living cells 

 as a ' mucous layer ' or ' primordial utricle, ' 

 but differing chemically from the substance 

 of the wall by being nitrogenous : this they 

 regarded as the living part of cell, and to it 

 Mohl (1846) gave the name 'protoplasm,' 

 which it still bears. The full significance 

 of this discovery became apparent in a 

 somewhat roundabout way. Dujardin, in 

 1835, had described a number of lowly 

 organisms, which he termed Infusoria, as 

 consisting of a living substance, which he 

 called ' sarcode.' Fifteen years later, in a 

 remarkable paper on Protococeus pluvialis, 

 Cohn drew attention to the similarity in 

 properties between the ' sarcode ' of the 

 Infusoria and the living substance of this 

 plant, and arrived at the brilliant generaliza- 

 tion that the ' protoplasm ' of the botanists 

 and the ' sarcode ' of the zoologists are iden- 

 tical. Thus arose the great conception of 

 the essential unity of life in all living things, 

 which, thanks to the subsequent labors of 

 such men as de Bary, Briicke, and Max 

 Schultze, in the first instance, has become 

 a fundamental canon of Biology. 



A conspicuous monument of this period 

 of activity is the cell-theory propounded by 

 Schwann in 1839. Briefly stated, Schwann's 

 theory was that all living bodies are 

 built up of structural units which are the 

 cells : each cell possesses an independent 

 vitality, so that nutrition and growth are 

 referable, not to the organism as a whole, 

 but to the individual cells. This concep- 

 tion of the structure of plants was accepted 

 for many years, but it has had to give way 

 before the advance of anatomical knowl- 



edge. The recognition of cell-division as 

 the process by which the cells are multi- 

 plied — in opposition to the Schleidenian 

 theory of free cell-formation — early sug- 

 gested doubts as to the propriety of regard- 

 ing the body as being built up of cells as a 

 wall is built of bricks. Later the minute 

 study of the Thallophyta revealed the ex- 

 istence of a number of plants, such as 

 Myxomycetes, the phycomycetous Fungi, 

 and the siphonaceous Algse, some of them 

 highly organized, the vegetative body of 

 which does not consist of cells. It became 

 clear that cellular structure is not essential 

 to life ; that it may be altogether absent or 

 present in various degree. Thus in the 

 higher plants the protoplasm is segmented 

 or septated by walls into uninucleate units 

 or 'energids' (Sachs), and such plants 

 are well described as 'completely septate.' 

 But in others, such as the higher Fungi and 

 certain Algse (e. g., Cladophora, Hydrodic- 

 tyon), the protoplasm is septated, not into 

 energids, but into groups of energids, so 

 that the body is 'incompletely septate.' 

 Finally there are the Thallophyta already 

 enumerated, in which there is complete 

 continuity of the protoplasm : these are 

 ' unseptate. ' Moreover, even when the 

 body presents the most complete cellular 

 structure, the energids are not isolated, but 

 are connected by delicate protoplasmic 

 fibrils traversing the intervening walls ; a 

 fact which is one of the most striking dis- 

 coveries in the department of histology. 

 This was first recognized in the sieve-tubes 

 by Hartig (1837) ; then by Naegeli (1846) 

 in the tissues of the Floridese. After a long 

 period of neglect the matter was taken up 

 once more by Tangi (1880), when it at- 

 tracted the attention of many investigators, 

 as the result of whose labors, especially 

 those of Mr. Gardiner, the general and per- 

 haps universal continuity of the protoplasm 

 in cellular plants has been established. 

 Hence the body is no longer regarded as 



