PLASTICITY OF SHOOT AND ROOT 2OI 



Care must be taken not to exaggerate the importance of 

 the node. It is not really a distinct structure which inci- 

 dentally produces a leaf, but it is the place where the leaf 

 stands and hence the fibre-vascular bundles of the stem 

 branch and anastamose, giving the " joint " appearance it 

 often, but by no means always, presents. The "phytomer" 

 has really no morphological existence, as I have elsewhere 

 pointed out (page 149), but is only an incidental result of 

 the way the stem is built. Moreover, this exercise should 

 make plain how readily the stem assumes the function of the 

 leaf, and how little distinct these two are from one another. 

 Hence the plant is best described as made up, not of leaf, 

 stem, and root, but of shoot and root, while the former is 

 further differentiated into stem and leaf, and the leaf may 

 be yet further specialized into blade, petiole, stipules. This 

 relation may be expressed as follows : 



! stem f Blade 



Higher Plant I Sho0t I Leaf Petiole 

 I Root c . , 



I Stipules 



(see, also, page 149). The main thing now is to teach the 

 fact of the existence of the different kinds of margin, shape, 

 etc., and to show how easily these are derivable one from 

 another, and to give some idea of their meaning. Leaf shape 

 may be treated in a lecture or demonstration somewhat after 

 this manner. The two extremes of shape possible are the 

 circle (accompanying fullest exposure to light) and the line 

 (where crowded), and between them are all variations of 

 ellipses, etc. When an intermediate form is borne out on a 

 long petiole, however, more material is condensed near the 

 petioles, and it gives forms like the ovate, etc. ; when they are 

 crowded together on a short stem so that they would shade 



