124 SCIENCE PROGRESS. 



changes. The same thing takes place when a root is con- 

 verted into a thorn (as in Myrmecodia echinata, or in 

 Acanthorhiza aculeatd), or into a storage receptacle for 

 reserve food. We see in all these, how the root cap 

 (calyptra) becomes lost, and the internal structure undergoes 

 modification, in accordance with the new functions which the 

 root is now called on to discharge. But these changes may 

 also be indicated before the root has actually begun to 

 exercise its normal office. Perhaps the most striking 

 examples of this are to be found amongst those roots 

 which come to serve as organs of carbon-assimilation. This 

 happens in several epiphytic orchids, as well as in that 

 remarkable aquatic order, the Podostemaceae. In certain 

 orchids, indeed, the roots are the sole assimilatory organs, 

 for the leaves have become degraded to mere colourless 

 scales (Angrczcum funale, Taeniophyllum sp.). And not 

 only do the roots become green, they may even assume the 

 dorsiventral structure of leaves, becoming flattened out on 

 the illuminated surface. I have observed, however, that in 

 the case of Taeniophyllum, the growing apex is still radial, 

 the flattened form being subsequently assumed during 

 development. 



Still more strikingly leaf-like are the roots of Dicrcea 

 algiformis, a plant which was carefully studied by Warming. 

 In every one of these instances, observation shows that it is 

 by no means in an " indifferent " mass of tissue, but in one 

 which is already clearly recognisable as appertaining to a root, 

 that the alterations of function and form have taken place. 



W T e may now turn and consider the leaf, and follow the 

 development of certain foliar tendrils, from the special point 

 of view immediately before us. And here we are at once 

 confronted by the question, as to whether the history of 

 development will enable us to arrive at a satisfactory con- 

 clusion as to the foliar nature of these structures. This 

 question may at once be answered in the affirmative. In- 

 deed, microscopic investigation is frequently unnecessary, 

 for the study of the seedling often affords evidence sufficient 

 for us. Thus, amongst the Fumariacese, in Corydalis, for 

 example, all intermediate stages may be observed. The 



