iS95. NOTES AND COMMENTS. 373 



The Cell in Botany. 



The gradual change in the idea " cell " is most plainly written in 

 Botanical History. Botanists retained the conception of the cell- 

 wall, combining with it Max Schultze's protoplasmic idea. In the 

 older text-books the plant unit is a " cell," i.e., a cellulose chamber 

 enclosing protoplasm and cell-sap. An aggregation of such, like 

 bricks in a wall, formed a tissue. The brick-wall metaphor was an 

 unfortunate one. It gave quite a wrong impression of the origin of 

 tissues, for the bricks are separate structures brought together to form 

 the whole, whereas the cells of a tissue have had a common origin. 

 Improvement in microscopic method has shown that, so far from being 

 independent units, cells are in many cases brought into intimate 

 connection with their neighbours by strands of the living protoplasm 

 passing through the separating wall. Further advance will probably 

 demonstrate such a connection between the cells in all actively living 

 tissues. 



The present position may be briefly stated as follows. The 

 unit is a mass of protoplasm in which is embedded a nucleus. Such 

 a unit, or " energid," as it has been termed, is the starting point of 

 every plant ; in the lowest forms, development goes no further — as, for 

 instance, in the smaller motile form of the tiny green alga which 

 examination syllabi call Pyotococcus. The first step towards a higher 

 development is the protection of the individual by a cell-wall, i.e., by 

 the secretion round itself of a cellulose membrane formed by and from 

 itself. The resting stage of " Protococcus," or the fertilised egg-cell, 

 the oospore of higher plants, is an example of such a unicellular 

 individual or phase. 



Further development may proceed in two ways. The proto- 

 plasmic unit (energid) may grow and divide repeatedly without the 

 separation of the resulting daughter units by partition walls. There 

 is thus produced within a common membrane, which undergoes 

 meanwhile a corresponding increase in size, a large number of nuclei 

 embedded in a general mass of protoplasm. This mode has been 

 adopted in whole groups of Algae and Fungi ; examples are the fresh- 

 water Vaiicheria and a well-known mould, Mucor. Such unseptate 

 individuals, which may reach a considerable size and show a sur- 

 prising amount of morphological differentiation, as in Caulerpa, where 

 a horizontal stem bears roots below and leaves above, are termed 

 " Coenocytes."' The structure in these cases may be strengthened 

 by bars of cellulose stretched across from wall to wall. 



Plants like the filamentous green fresh-water alga Cladophora 

 illustrate incomplete septation. Each segment of the filament 

 includes within its wall, not one, but a number of "energids." In most 

 cases where the individual attains any size and must provide its own 

 support, as in land-plants, the completely septate form has been 

 adopted. Here the protoplasmic units are each separated (though 

 probably not isolated) by a cell-wall. Thus in the great majority of 



