312 MODIFIED CIECUMNUTATION. Chap. VI 



In our entire list of seedlings, there are 30 genera, 

 belonging to 16 Families, the cotyledons of which in 

 some of the species rise or sink in the evening or 

 early night, so as to stand at least 60° above or be- 

 neath the horizon. In a large majority of the genera, 

 namely, 24, the movement is a rising one; so that 

 the same direction prevails in these nyctitropic move- 

 ments as in the lesser periodic ones described in the 

 second chapter. The cotyledons move downwards 

 during the early part of the night in only 6 of the 

 genera; and in one of them. Cannabis, the curving 

 down of the tip is probably due to epinasty, as Kraus 

 believes to be the case with the leaves. The down- 

 ward movement to the amount of 90^ is very decided 

 in Oxalis Valdiviana and sensiiiva, and in Geranium 

 rotundifoKum. It is a remarkable fact that with Anoda 

 Wrightii, one species of G-ossypium and at least 3 

 species of Ipomoea, the cotyledons whilst young and 

 light sink at night very little or not at all ; although 

 this movement becomes well pronounced as soon as 

 they have grown large and heavy. Although the 

 downward movement cannot be attributed to the 

 weight of the cotyledons in the several cases which 

 were investigated, namely, in those of the Anoda, 

 Ipomoea purpurea and bona-nox, nor in that of I coo- 

 cinea, yet bearing in mind that cotyledons are con- 

 tinually circumnutating, a slight cause might at first 

 have determined whether the great nocturnal move- 

 ment should be upwards or downwards. We may 

 therefore suspect that in some aboriginal member of 

 the groups in question, the weight of the cotyledons 

 first determined the downward direction. The fact of 

 the cotyledons of these species not sinking down much 

 whilst they are young and tender, seems opposed to 

 the belief that the greater movement when they .are 



