HOW PLANTS GROW. 217 



Take what plant, or what part of any plant, you choose, it is 

 wholly made up of cells, and nothing but cells. These may be 

 modified so as to produce " vessels " ; they are still only cells. 



If only the simple cells be present, we call the plant cellular ; 

 as, for example. Seaweeds, Lichens, Moulds, Liverworts, and 

 Mosses. If, in addition to these simple cells, we find modified 

 kinds, or vessels, we call it vascular^ including the Club-mosses, 

 Horse-tails, Ferns, and all Flowering plants. 



Remembering this axiom, which, unlike the Euclidian axioms, 

 is capable of being demonstrated under the microscope, and is 

 not, on the face of it, always '' self-evident " without this pro- 

 cess, let us, to start with, reply to the query, " What is a cell ? " 



ia) The Cell as an Individual. 



Until quite recently, a cell was supposed to consist essentially 

 of a cell-wall enclosing certain contents, such as protoplasm, cell- 

 sap, nucleus, etc. The cell-wall was held by some to be the 

 essential feature ; indeed, many observers used the term ''cell" to 

 denote cases where only the cell-wall existed, the former contents 

 having vanished, as, for instance, pith-cells and very aged cells, 

 where only air is contained by the wall. Now, however, many 

 cells are known to exist, for part of their life at all events, witJioiit 

 any wall differi?ig chemically from their other substance. There 

 do also exist some cells devoid of a nucleus, so far as we can 

 ascertain. Hence, some modification of the scope of the term 

 " cell " was needed, and now, universally, the only essential consti- 

 tuent of a plant-cell is held to be the protoplasm, a viscid, semi- 

 fluid substance. A cell^ therefore, pure and simple, is simply a 

 mass of protoplasm. This protoplasm is always, sooner or later, 

 closed in by a solid, more or less firm, transparent membrane, 

 secreted by itself, and moreover possesses, in most cases, a nucleus. 

 Still, the one great essential feature is protoplasm. Only cells 

 which have it are " alive " ; only such cells can grow, change, 

 secrete other substances, or reproduce themselves. Destitute of 

 it, they are " dead," and can only subserve the uses of support, 

 extension of fabric or framework, attraction of fluid, or protection. 



Let us now study a typical cell^ having not only the living, 

 acting protoplasm, but the usual concomitants of cell-wall, nucleus, 

 and other parts. 



