4 o PHYSIOLOGICAL MORPHOLOGY 



and division of labour takes place, which may be exhibited externally in 

 the morphological characters and peculiarities of the component members. 

 Owing to the functions which its peripheral position allots to it, the epidermis 

 must acquire properties that vary according to the nature of the organ it 

 covers; similarly, the different parts of the vascular bundles have very 

 different tasks to perform. Very important also are dead elements, such 

 as compose the skeletal framework of all the higher plants, and are found in 

 each protective cork layer, while intercellular spaces also have important 

 functions. 



When organs and tissue elements are fully developed, and have 

 attained their full anatomical and morphological differentiation, an adult 

 condition is reached, which persists for a longer or shorter period. To 

 permit of continued growth, and to ensure the possibility of reproduction, 

 embryonic tissue must be preserved in certain regions. It is in virtue of 

 the presence of such embryonic tissue at its apex, that the shoot continues to 

 grow in length, while the meristematic cambial ring enables a tree to add 

 every year a new layer of wood. On the other hand, in the leaf, the loss of 

 the primary meristem involves an ultimate cessation of growth. 



The life of every adult and functionally active cell is, as far as we 

 know, of limited duration. In every tree, year by year, the older cells 

 die, so that in the stems of trees hundreds of years old, only a certain 

 number of the youngest annual rings contain living elements. It appears 

 as if the protoplast can only continue to exist by unceasing growth and 

 division, and unless such regeneration takes place, it becomes exhausted, 

 and ceases to be functionally active, just as a machine wears out by con- 

 tinued use. A bacterium cell is in a certain sense immortal, so long as it 

 can grow and divide. If its growth and division were mechanically 

 arrested, however, it would in all probability finally die, although all other 

 conditions might be suitable. This is actually the case in the primary 

 meristem of the root and shoot, for these ultimately die 1 when embedded 

 in a plaster of Paris cast, though they may remain living for some time. 



Where, however, adult organs capable of remaining active for a given 

 length of time are required, each organ and each of its component cells 

 must pass through a series of developmental phases, leading gradually to 

 the final form. The questions connected herewith have already been 

 noticed from a physiological standpoint 2 . 



Relation of morphology to Physiology. Although our purpose is specially directed 

 to the consideration of functional aspects, it seems, nevertheless, most convenient to 

 adhere to the morphological distinctions and anatomical nomenclature used in the 

 textbooks of Sachs, De Bary, and Strasburger. The characters which determine 



1 Pfeffcr, Druck und Arbeitsleistung, 1893, p. 355. 

 ' See Sachs, Flora, 1893, p. 323. 



