98 SCIENCE PROGRESS. 



questions still remain, is there some primary form of leaf 

 of which all the others may be regarded as modifications ? 

 and, if so, what is that form ? The first of these questions 

 must, as it seems to me, be answered in the affirmative ; 

 for material metamorphosis necessarily involves such an 

 assumption. We have traced a series of forms of leaves, 

 differing the one from the other by the characters which 

 they have assumed as the result of adaptation to their several 

 functions : the cataphylls to protect the buds ; the foliage- 

 leaves to discharge certain nutritive functions ; the sepals 

 to protect the flower ; the petals to attract the visits of 

 insects ; the stamens and carpels to bear the reproductive 

 cells ; and this series must have had a definite starting-point. 

 In endeavouring to determine this starting-point, to discover 

 this primitive form, it is evident that two alternatives are 

 open to us ; the series has two ends which we may dis- 

 tinguish as the higher and the lower, and it is obviously 

 possible to take one or the other of these ends as the 

 starting-point, to regard either the sporophyll or the 

 foliage-leaf as the primitive form. Goethe long ago 

 was struck by the fact that both ascending (from the foliage- 

 leaf upwards) and descending (from the sporophyll down- 

 wards) metamorphoses are exhibited by plants. Which of 

 these modes is the course actually followed in the differ- 

 entiation of the organs of the plant ? 



As a matter of fact, both these possible views have 

 their supporters ; and the point may be regarded as still 

 open to discussion. There can be no doubt that those who 

 first introduced the idea of material metamorphosis, basing 

 themselves upon the ascertained fact that the leaves are 

 developed in acropetal succession at the growing-point of 

 the stem — so that the youngest leaves are nearest to, the 

 oldest most remote from it — adopted the theory of ascending 

 metamorphosis, regarding the foliage-leaf as the primitive 

 form of which all the others are modifications as the result 

 of special adaptation to their various functions. In the 

 present day the strongest supporter of this view is Goebel, 

 whose position may be gathered from the following quota- 

 tion : " Herein lies the morphological equivalence of the 



