248 BOTANY I-AKT i 



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

 so that, according to SACHS, three chief phases of growth can be 

 distinguished, which, however, are not sharply separated, but merge 

 imperceptibly one into the other : 



1. The embryonic phase in which the rudiments of new organs 

 are formed. 



2. The phase of elongation of the already formed embryonal 

 organs. 



3. 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 differentiation of the tissues has already taken place. 



Leafy shoots may also take their origin from old and fully 

 developed tissues, which again assume an embryonic character, 

 accompanied by an accumulation of protoplasm and renewed activity 

 in cell division. But as this only occurs in exceptional cases, shoots 

 which thus arise out of their regular order are termed ADVENTITIOUS. 



The manner of the Formation of New Organs at the Growing 

 Point has already been described in the morphological portion of this 

 book. It is only necessary here to again call attention to the fact 

 that the young organs, with few exceptions, develop in acropetal 

 succession, so that the youngest is always nearest the apex. The 

 point from which new organs arise, and the number which develop, 

 are chiefly dependent upon inherited internal disposition. External 

 factors can, however, exert an effect in particular cases. The 

 influence of such factors as light, gravity, chemical and mechanical 

 stimuli, which at certain times in the later life of the tissues is of 

 extreme importance, has usually but little effect on the embryonal 

 development. Yet, on the other hand, the position of the first 

 division wall of the germinating spore of Marsilia is determined by 

 the action of gravity, and the direction of the first wall (as well as 

 of the preceding nuclear division) in the spore of Equisetum and the 

 ova of Cystoseira barbata, Pelvetia, Haliseris, and Ascophyllum among 

 the Fucaceae, and of Dictyota is determined by its relative position to 

 the light ( 62 ). 



In Adventitious Formations, on the contrary, the influence of 

 external forces is often very evident, as, for example, in the formation 



