250 BOTANY pakt i 



Ccan be termed growth, and this whether the plant as a whole is 

 gaining or losing in substance. Usually growth is associated with 

 gain of material, but in the case of potatoes sprouting in a dark cellar 

 loss takes place by transpiration and respiration, and yet the shoots 

 exhibit growth. 



So long as a plant lives it exhibits growth if the necessary external 

 conditions are present. If these are wanting, the plant may lead a 

 "latent" life in which it retains the capacity for development. This 

 latent life does not last indefinitely ; even seeds, which exhibit it most 

 completely, lose their power of development in the course of months 

 or years and die. Particular organs in many plants lose their power 

 of further development more or less completely ; they become full- 

 grown and sooner or later perish. 



In the simplest plants, such as the lower Algae, Fungi, or Bacteria, 

 development consists merely in growth of the cell followed by cell- 

 division. These cases have been sufficiently dealt with in the morpho- 

 logical section. In more complex plants growth and division of cells 

 are also found, but these processes appear subordinated to the growth 

 of the whole. In this three distinct processes can be distinguished, 

 though they are not always separated in time. These are the stage of 

 FORMATION OF EMBRYONIC ORGANS, that of ELONGATION, and the stage 

 of INTERNAL DEVELOPMENT {^^). These processes must be considered 

 separately and their dependence upon a variety of factors discussed. 



I. The Embryonal Development of the Organs 



Plants, in contrast to the higher animals, continually develop new 

 organs. These arise either from tissues retained in their embryonic 

 condition, as at the growing point, or they have their origin in regions 

 which have already more or less completely attained their definite 

 form. The leaves and shoots spring directly from the tissues of the 

 growing point ; the first lateral roots, however, make their appearance 

 at some distance from the growing point, where a perceptible 

 diflferentiation of the tissues has already taken place. 



Leafy shoots may also take their origin from old and fully 

 developed tissues, which then resume an embryonic character, accom- 

 panied by an accumulation of protoplasm and renewed activity in cell 

 division. Organs which thus arise out of their regular order are termed 

 ADVENTITIOUS. As a rule, however, tliey develop from the growing 

 point, either at once or after a longer or shorter period of rest. 



'^The manner of the Formation of New Organs at the Growing 

 Point has already been described in the morphological portion of this 

 book. In the development of the embryo of the higher plants, two 

 growing points, one of the shoot and the other of the root, are produced. 

 A corresponding polar difterentiation into base and apex also occurs in 

 much more simply constructed plants, and even in single cells. Since 



