30 THE LIFE OF THE PLANT 



cohesion of cells to be convinced of the fact that vegetable 

 matter is composed of them : if we cut very thin and 

 transparent slices with a razor from any part of a plant 

 we can soon satisfy ourselves with the help of a micro- 

 scope that these are composed of cells, closely compacted 

 together, forming what is known as cellular tissue. 



It is clear from what has been said that it is im- 

 possible to become acquainted with the structure and 

 life of plant organs without an acquaintance with the 

 cell. As in chemistry we start the study of substances 

 with the elements and then proceed to their combina- 

 tions, so in botany the study of the organs of plants 

 must be preceded by that of their elementary organ — the 

 cell. 



We have now collected enough facts to be able to 

 make a general plan for these lectures. During its life- 

 history the plant produces a series of organs, the 

 external aspect of which, together with their relation 

 towards their environment, makes it evident that 

 they serve very different purposes and perform very 

 different functions. It is clear that the function of 

 the root which sinks into the soil is different from 

 that of the green leaf which grows up into the air 

 towards the light ; that the function of the cotyledon is 

 different from that of the petal ; that the function of the 

 stamen with its pollen so easily disseminated in the air 

 is not the same as that of the ovule buried deep in the 

 ovary. The physiologist first of all must discover the 

 purpose of every organ, i.e. its function. Hence a 

 twofold problem confronts him from the outset : given 

 an organ, to find its function ; and given a function, to 

 find the organ. Evidently he has first to study the 

 function of the elementary organ, the cell, in its general 

 and special manifestations. Later on, when he becomes 

 convinced of the perfect way in which the organs fulfil 

 their purpose and are adapted to their environment. 



