TWENTY-THIBD ANNUAL MEETING. 169 



the leaves of the rose and those of the oak. But besides the difference existing be- 

 tween orders, genera and species, there is also often a marked variation in the 

 leaves of one and the same plant: and it is this particiilar phase of variation to 

 which 1 wish to call attention. 



One of the most familiar plants which furnishes an illustration of this variation 

 among its individual leaves is rubus villosus, common high blackberry, which usually 

 has from three to five leaflets arising from a common point on the footstalk. But 

 many of the leaves do not answer to this description. As will be seen by referring 

 to figures 1 11, the two lower leaflets are often found to be more or less lobed in the 

 trifoliate leaves. Upon closer examination, such a prodigious number of leaves 

 exhibited this peculiarity that I was led to consider them as representing transition 

 stages between three and five leaflets. The highest type of the present seems to be 

 five leaflets, and the threes apjiear to be struggling to reach that number. To make 

 sure that the leaves first observed were not '• freaks of nature," or from a botanical 

 .standpoint, monstrosities, the whole neighboring region was explored, and every- 

 where the blackberry leaves exhibited indications of this peculiarity, which I have 

 chosen to designate as an evolutionary tendency. The first indication of the division 

 or transition of a leaf, according to my observations, is an enlargement on the por- 

 tion of the leaf where the division is about to take place; further on in the series, 

 this enlargement becomes a conspicuous bulge, as shown in figure 6, and the vein 

 which is to form the midrib of the newly-formed leaflet becomes strong and prom- 

 inent: a notch is next formed, which deepens into a lobe, and finally the entire 

 leaflet is given off, as shown in figures 7, 8 and 9. For instance, in the trifoliate 

 leaves of the blackberry, the enlargement will be found on the lower part of the two 

 lateral leaflets, then the various stages of division, resulting finally in the five 

 leaflets. Single leaves were also found in process of division into three leaflets, and 

 two specimens were found bearing six leaflets; one in which the terminal leaflet had 

 thrown off an additional leaflet, and the other with the extra leaflet given off from 

 one of the lowest lateral leaflets. (Figs. 10-11.) My attention being aroused in 

 this direction, I now noticed in very many plants similar transition stages. The 

 amjjelopsis quinquefolia, Virginia creeper, afforded numerous illustrations. The 

 normal number of leaflets, as the name indicates, is five, but the transition from 

 five to six and seven leaflets was noted in very many cases; first, an enlargement 

 was observed on the lower or outer portion of the lower leaflets, then a notch, 

 which set forth. One should be able to find transition stages in the cinquefoil: /. e., 

 cases where the division into five leaflets was still in process. 



Carlyle says. "How few people think: aye, reader, how few people think "; and we 

 may add, how much there is which we fail to see. How many times before I had ex- 

 amined and admired the leaves of this potentilla. and yet had never appreciated the 

 embryonic history of evolution stamped upon it, for it still retains the single leaflet, 

 perhaps the ancestral type, and transition forms from one to five leaflets are readily 

 found. All pinnate leaves which were observed, except those of the rose, continue 

 to form new leaflets by a division of the terminal leaflet: deepened as the series con- 

 tinued, until the seven perfect leaflets were formed. The Rhus Toxicodendron. Poi- 

 son Ivy. is so often sjioken of in contradistinction to the ampelopsis quinquefolia. 

 that a study of its leaves was next made. The Rhus Toxicodendron commonly has 

 three more or less entire leaflets, though they are often sinuate or cut-lobed. A 

 number of specimens were found where the lower leaflets had thrown off another 

 pair, making it five-leaved. The tendency, however, seems to be to develop toward 

 nine leaflets rather than five, each of the three leaflets becoming trifoliate. Poten- 

 tilla Canadensis, the common cinquefoil, next claimed my attention. According to 

 the ideas here, when a new leaflet is about to be formed the terminal leaflet will often 



