THE STEM OB SHOOT. 



125 



obviously of reversionary nature, viz., when the dorsi- 

 ventral climbing shoot of ivy becomes erect and 

 radially-symmetrical, and when that highly modified 

 organ, the rhizophore of Selaginella, becomes changed 

 into a leafy shoot. 



Farther, it is evident that shoots may grow in 

 any direction, the sole criterion being the particular 

 needs of the shoot concerned. There exists no rule 

 or law, however general, according to which a shoot 

 grows away from the centre of the earth under the 

 influence of gravity. The statolith theory does not 

 provide the regulative cause of the direction of growth, 

 but only a reminding stimulus which is subsidiary to, 

 and, in origin, cannot be coeval with, the primary 

 cause, which is the habit of the shoot, ingrained in its 

 substance, to grow in this or that direction ; but if the 

 normal external stimuli, e.g. moisture or light, are want- 

 ing or greatly modified, that will necessarily cause a 

 modification of the growth-direction, simply because 

 the shoot then meets with conditions to which it is 

 unaccustomed. 



The misleading way in which most modern physio- 

 logists write about the influence of gravity is well 

 illustrated by a sentence which may here be quoted 

 from a recent text-book. After speaking of the way 

 in which unicellular Algae move towards a source of 

 oxygen, and in which spermatozoa swim towards the 

 ovum under chemical influence, the author says : " The 

 growth of roots towards the centre of the earth and 

 of shoots away from it is due to a perception of the 

 direction of the gravity-pull." * Any ordinary student 

 reading this would surely conclude the author's meaning 

 to be that the root as a whole and the shoot as a whole 

 are influenced directly by the gravity-stimulus in the 

 same way as the Algae and the spermatozoa previously 

 cited are directly influenced by the oxygen and chemical 

 substances respectively. This, of course, is not so. 



* Stevens/ Plant Anatomy in its Relation to Development and Function. ' 



