458 LIFE : OUTLINES OF GENERAL BIOLOGY 



ously growing fruit-trees have often to be root-pruned, because the 

 check to nutrition favours the reproductive activity which we 

 desire, as flowering and fruiting. But if fohage and vegetative 

 activity is desired, then the flower-buds have to be nipped off to 

 prevent "running to seed". Similarly it is well kno\\'n that castrated 

 domesticated animals tend to put on flesh and fat — whence ox, 

 capon, and the like. Nutrition and reproduction are thus substan- 

 tially antithetic. It is not accidental, but fundamental, that the flower 

 oftenest occurs towards the apex or at least well up the vegeta- 

 tive axis, where food-supplies are at a minimum, though this con- 



FiG. 59. 



The Moonwort Fern {Botrychium lunare), showing the contrast between the 

 vegetative frond {'i) and the spore-bearing fructification (6). After 

 Sachs. 



sideration is too simple as we have stated it for the moment; for 

 it requires, for instance, to be also correlated with the occur- 

 rence of what arc called "metabolic g~adients". 



METABOLIC GRADIENTS.— In Planaria, a simple Turbellarian 

 worm of ponds and streams, the everyday chemical routine or 

 metabolism, as measured by the COj output, etc., is most intense 

 at the anterior end of the body, and gradually wanes to a point 

 distant from the head by about three-quarters of the total length. 

 There the regulative influence exerted by the head is at a minimum, 

 and it is at this point that preparation begins to be made for asexual 

 reproduction and for the development of a second head, which will 



