70 Ve(jetahle Phydologij. 



In perennials the flowering' and fruiting^ may be repeated jear 

 after year, and in trees the life is capable of extension to an in- 

 definite period, apparently only limited by external circumstances. 

 In a long course of existence the tree does not absorb and excrete 

 the dead and worn out structures like an animal, replacing them 

 in the same spot and in the same condition, but in part throws 

 them off entirely, as in the falling leaves and the withering en- 

 velopes of the blossom, replaced on new shoots springing forth 

 beyond them — in part overgrows and buries them, as it were, 

 retaining them as a solid foundation for younger growth, as when 

 the heart-wood of the oak is increased by yearly layers of new 

 substance, or the crown of the palm is gradually elevated upon 

 its monumental column. 



The study of development is the great business of the vege- 

 table physiologist. But the study of comparative physiology is 

 scarcely less important when guided and checked by the other 

 branch of research. For, as is known to every one who has 

 mastered the rudiments of natural history, the organic kingdoms 

 present us with countless different kinds of plants and animals, 

 in which we recognise almost every possible different degree of 

 complexity (or simplicity) of organization. And it is also well 

 known that the higher forms all pass through stages which, 

 although actually very different and with a different destiny, may 

 be compared, as regards the physiological phenomena they pre- 

 sent, to different perfect kinds standing fixed at successive poiiits 

 of elevation in the scale of organization. 



The kinds belonging to tlie lower classes of animals and 

 plants, from the greater simplicity of structure, admit of our 

 examining them more completely and thoroughly in a living 

 state. It is manifest that we could not observe the conditions of 

 structure of a leaf or other organ of the higher plants without 

 dissection and consequent destruction. But there exist plants of 

 small size and simple organization, composed of merely a few 

 cells, the organic elements of which the leal is composed. These 

 minute forms of life are so small and transparent, that we can 

 see through and through them by the help of the microscope. 

 Therefore, when anatomy proves to us that the tissues are similar, 

 and chemistry tells us that the combinations and decompositions 

 which take place in them are the same, we fairly conclude that 

 our observations of the phenomena of growth and reproduction 

 in these lower plants afford us sound data for ascertaining the 

 laws which govern the life of the higher forms. Nature thus 

 not only gives us, as it were, dissections ready-made, but she ex- 

 hibits, as it were, fragments of life from which we may piece 

 together tl.e complicated sum of the life of the higher forms. 



The pursuit of the development of the higher forms, from the 



