NUTRITION: THE WORK AND THE MATERIALS. 7 



all that we can tangibly realise of the manifestations of 

 life. 



With few .exceptions, which it is not necessary here to 

 particularise, the protoplasm does not exist in one un- 

 broken mass, but is contained in little membranous bags 

 called "cells." These cells are of various shapes and 

 sizes, and may undergo various modifications during the 

 growth of the plant. They are large enough to be seen by 

 the naked eye in the pulp of an orange, but usually they 

 require the aid of the microscope to discern them. They 

 are lengthened into tubes, placed end to end to form con- 

 duits, or thickened into fibres. The cells, then, variously 

 combined and modified, constitute what we have termed 

 the fabric of the plant. Each living cell consists essentially 

 of a certain proportion of protoplasm contained within a 

 membranous bag or bladder, called technically the " cell- 

 wall." There may be, and generally are, other things 

 besides the protoplasm within the cell-wall, such, for in- 

 stance, as a small ovoid body known as the "nucleus," and 

 green colouring matter or " chlorophyll ; " but these other 

 things, important as they are, we may leave out of con- 

 sideration for the present. 



Every plant and every part of every plant is made up of 

 cells such as have been mentioned. As a cell a plant 

 begins its independent life; with and by cells it lives, 

 grows, multiplies ; by their decay it dies. It is, as has 

 been said, the protoplasm, which is the essential agent in 

 all these processes ; but, subject to a few exceptions, which 

 need not now be specified, this protoplasm is always shut 

 up within a cell-wall. Nor is it absolutely necessary that 

 there should be more than one cell. Most plants with 

 which the cultivator has to do consist of aggregations of 



