PHYSIOLOGY 225 



Leafy shoots may also take their origin from old and fully- 

 developed tissues, which again assume an embryonic character, accom- 

 panied 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 (p. 149). It is only necessary here 

 to again call attention to the fact that the young organs develop in 

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

 This is, in fact, the most natural method in consideration of the apical 

 growth of the axes. In spite of that, however, special cases are 

 known in which the young organs arise at some distance from the 

 growing point, and between older organs (in the inflorescence of 

 Typha). The point from which new organs arise, and the number 

 which develop, are chiefly dependent upon inherited internal disposi- 

 tion. Although external conditions exert in this respect but small 

 influence, it has been recognised that the available space, and the 

 subsequent pressure of the older organs of the vegetative cone, as 

 well as the torsion of the axis, operate in determining the ultimate 

 position of new organs on the parent axis. The influence of other 

 factors, light, gravity, chemical and mechanical stimuli, which at certain 

 times in the later life of the tissues are of extreme importance, have 

 usually but little effect on the embryonal development. Yet, on 

 the other hand, the position of the first division wall of the ger- 

 minating 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 is determined by its relative position 

 to the light. 



In Adventitious Formations, on the contrary, the influence of 

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

 of climbing-roots, which in the Ivy and other root-climbers are de- 

 veloped only on the shaded side of the stem. In the Alga Gaulerpa 

 the new leaf-like organs arise only on the illuminated side of the parent 

 organ. It is, on the other hand, the force of gravity which excites 

 the formation of roots on the under side of underground rhizomes. It 

 is also due to gravity that the growing points of shoots are formed only 

 from the upper side of the tubers of Thladiantha dubia, or that new 

 twigs develop, for the most part, from the upper side of the obliquely 

 growing branches of trees. Contact stimuli, on the other hand, 

 determine the primary inception and point of development of the 

 haustoria of Cuscuta (p. 208). The sexual organs of Fern prothallia 

 are always developed on the side away from the light ; that is, in 

 normal conditions on the under side, but in case of artificial illumina- 

 tion on the upper side. 



As a result of one-sided illumination and the stimulus of gravity, 

 together with the favouring influence of moisture, the rhizoids spring 



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