150 THE TEACHING BOTANIST 



Among errors very dear to us is the belief that 

 monstrosities are reversions to an earlier condition, and 

 hence good guides to the past history of organs or 

 species. It is true they may be, and of course often 

 are ; but they so frequently are not that great caution 

 must be exercised in using them as guides to phylogeny. 

 If the turning of a petal green is taken to prove that 

 the petal was once a foliage leaf, then the turning red 

 or yellow of the leaf under the flower of a tulip must be 

 taken to prove that this leaf was once a petal, which is, 

 of course, not to be believed ; hence the turning green of 

 petals means nothing more than a disturbance of nutri- 

 tion conditions. This principle applies to the cases 

 where carpels become leaves and the ovules leaf-like 

 bodies, which need not mean that these were once of a 

 green leaf nature, but only that the plant has for some 

 reason unknown made its materials build green leaf 

 tissue instead of carpellary tissue at that place. 



Very common and serious are physiological errors, of 

 which perhaps the most widespread is the belief that 

 animals and plants are the exact opposites of each 

 another with reference to the taking in and giving off 

 of the two very important gases, carbon dioxide and 

 oxygen. In a general way this is true, but not in the 

 sense in which it is usually meant. In fact, in all of 

 their processes of growth, movements, etc., animals and 

 plants behave precisely alike with reference to these two 

 gases, in both cases taking in oxygen and giving out 



