Vl PRESIDENTIAL ADDRESS 



generalisation that all animals and plants consist of a cell or 

 cells — the lowest bein^' constituted by a single cell, and the more 

 complex being built up of a number of cells, either all alike and 

 without differentiation, or in the higher organisms modified and 

 adapted to subserve a variety of functions. What the cell is, 

 what are its contents, what is their relative importance, and 

 why it is the seat of life, are questions which have passed 

 through such changing phases that now the very word " cell " is 

 seen to be a misnomer based upon a half-truth, and although 

 the word is retained it is only because it has taken its place in 

 scientific nomenclature, and is now used as a term whereby to 

 designate the biological unit. In the latter part of the seven- 

 teenth century the study of plant structure led to the observation 

 of minute S[;aces or compartments, provided with firm walls in 

 some cases, empty in others, filled or partly filled with fluid 

 contents. These little compartments, which are so minute that 

 they can only be seen under the microscope, received the name 

 of cells, and the term cell had every appearance of appropriate- 

 ness, for in plants these units appear to be minute vesicles, each 

 surrounded by its own wall. This was the view promulgated by 

 Schleiden in 18B8, and he declared that plants are made up of 

 cells, and that their vessels are modifications of cells. ^V'hy 

 Schleiden reached such a conclusion will be obvious if we 

 examine some examples of cells. If we take, for instance, a 

 small portion of a seaweed and examine it microscopically under 

 & magnification of about 100 diameters, it will be observed that 

 it is built up of minute compartments, each surrounded with a 

 ■comparatively thick wall. A similar structure is characteristic 

 of plants generally. The thickened cell walls are well seen if we 

 examine a piece of the substance known as rice paper, which is 

 obtained by sections of the herbaceous stem of the Chinese plant 

 Aralia. The cuticle of a leaf also affords a typical illustration. 

 Stripped off and examined under a power of about 150 diameters, 

 not only the cells of the epiderm are seen, but the peculiar modi- 

 fied cells known as guard cells, which flank the pores or stomata 

 of the plant. These cells, though altered in shape by their 

 adaptation as supports, nevertheless present the same appearance 

 of minute vesicles. If we make a transverse section of the leaf, 

 froin surface to surface, and magnify it to about the same extent, 

 we see the same cellular structure varying from the more com. 

 pact cells of the epiderm to the more expanded cells of the 

 interior, and a transverse section of a stem or twig presents the 

 same cellular formation. In the pith of a rush the cell walls 



