2 LECTURE T. 



the most important characteristics of the vegetative organs. In this I do not aim in 

 any way at setting forth, in the sense of descripti\'e or morphological Botany, all 

 possible varieties of form of homonymous organs in the whole vegetable kingdom ; 

 the point in question here is rather to show the beginner, proceeding from physio- 

 logical points of view, the most important modifications of form, since the purely 

 formal morphological contemplation of the organs of the plant customary hitherto 

 has left their physiological relations entirely out of account. Any one who has 

 been exclusively concerned with the formal morphology of the vegetable kingdom 

 prevalent during the last thirty or forty years, can scarcely concei\e the importance 

 which the vegetative organs possess physiologically. 



Considering the grand variety of forms in the vegetable kingdom, which com- 

 mences with simple spheroidal organisms only visible with the microscope, and 

 ascends to such forms as we meet with in phanerogamous trees and shrubs, it 

 is not easy to state in a few words the correspondences in nature of similar organs, 

 and their essential differences. We know, it is true, that from the lowest stages of 

 vegetable organisation up to the most highly developed plants, the same plan of 

 organisation is always adhered to in all essential points. It is found, however, that 

 very often organs which are in their nature of the same kind, may be endowed with 

 not only very different forms, but also with different physiological functions. Since 

 we assume from the standpoint of the theory of descent that the more highly 

 organised forms have been developed from the simpler, the most varied shapes 

 from the same primary form, it becomes clear that very different organs may 

 nevertheless be of like original nature ; but that, in the course of changes which the 

 whole organisation of the vegetable kingdom has undergone in time, they have been 

 able to obtain new qualities, and to lose older ones. Thence follows, however, that 

 we are not in a position to say in a few words which properties of an organ are 

 essential, and correspond originally with the general plan of construction of the 

 vegetable kingdom; in other words, it is not possible to express organographical 

 ideas clearly and exhaustively by means of simple definitions. 



We adopt, therefore, a totally different mode of consideration. Without concerning 

 ourselves in any way with definitions, we regard first the various organs where they 

 present themselves in the highest perfection in their typical character, and then seek 

 to establish which organs, in other regions of the vegetable kingdom, present also the 

 same essential peculiarities more or less modified. In doing this, however, we place 

 in the foreground the physiological properties, which very often correspond but litde 

 with the relations of outward form which constitute the subject-matter of morphology. 

 I believe, however, that this comparative physiological method of consideration of the 

 organs apprehends their true nature in a more fundamental manner than morphology 

 has done hitherto. 



Above all, physiological organography has to do, not merely with the incidental 

 visible forms of organs, but more especially with their functional importance for the 

 life of the plant; these functional activities, however, are nothing but reactions to 

 external influences, even where the case appears to be otherwise. Each reaction 

 of an organ to external influence is, however, what is usually termed irritability, as 

 will be shown more fully later on ; all the organs of the plant, without exception, are 

 in some one sense irritable, and the essential differences and agreements of the 



