224 BOTANY 



microscopically small and of the simplest structure, To attain its 

 final size and perfect development it must grow, that is, it must 

 enlarge its body and undergo differentiation. Even for the minute 

 unicellular bacteria growth is essential, as they multiply chiefly by 

 cell division. Each daughter cell must grow and attain the 

 dimensions o£ the parent cell, or in a few years the capacity for 

 existence itself will be lost through their continually decreasing size. 

 It is in fact impossible to conceive of a plant where perfect de- 

 velopment is not the result of growth. If a growing Oak or Cedar 

 be compared with the single spherical egg- cell from which it has 

 arisen, it is at once clear that by the term growth we mean not 

 only an increase in volume, but include also a long series of 

 various developmental stages, and external and internal modi- 

 fications. A mere increase in volume does not necessarily imply 

 growth, for no one would say that a dried and shrivelled turnip grows 

 when it swells in water. On the contrary, active growth may be 

 accompanied by a considerable loss of substance, as is shown by the 

 sprouting of potatoes kept in a dark cellar. Water is lost through 

 transpiration as well as organic substance through respiration, and yet 

 the new shoots show true growth. 



In the lower organisms growth is exhibited in its most simple 

 form. In an Amceba or a Plasmodium growth is simply an increase 

 in their substance ; in a unicellular Alga or in a Fungus it means, in 

 addition to this, an enlargement of their cell walls. In the higher 

 plants the processes of growth are far more complicated and various, so 

 that, according to Sachs, four chief phases of growth can be distin- 

 guished, which, however, are not sharply separated, but merge imper- 

 ceptibly one into the other. 



1. The embryonic phase, or the first origination of new cells or 

 organs, according to their proper position and number. 



2. The formative phase, or the continuance of the embryonal 

 development, and the assumption of a definite form. 



3. The phase of elongation of the already formed embryonal 

 organs. 



4. The phase of internal development and completion of the tissues. 



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 differen- 

 tiation of the tissues has already taken place. 



