CHAP. II. PULVINI OF COTYLEDONS 121 



"We here see that the cells of the pulvinas increase but little 

 iu length with advancing age, in comparison with those of the 

 petiole both above and . below it ; but they continue to grow in 

 width, and keep equal in this respect with the other cells of 

 the petiole. The rate of growth, however, varies in all parts 

 of the cotyledons, as may be observed in the measurements of 

 the 8-days' old seedling. 



The cotyledons of seedlings only a day old rise at night con- 

 siderably, sometimes as much as afterwards; but there was 

 much variation in this respect. As the pulvinus is so indistinct 

 at first, the movement probably does not then depend on the 

 expansion of its cells, but on periodically unequal growth in 

 the petiole. By the comparison of seedlings of different known 

 ages, it was evident that the chief seat of growth of the petiole 

 was in the upper part between the pulvinus and the blade; 

 and this agrees with the fact (shown in the measurements above 

 given) that the cells grow to a greater length in the upper than 

 in the lower part. With a seedling 11 days old, the nocturnal 

 rise was found to depend largely on the action of the pulvinus, 

 for the petiole at night was curved upwards at this point ; and 

 during the day, whilst the petiole was horizontal, the lower 

 surface of the pulvinus was wrinkled with the upper surface 

 tense. Although the cotyledons at an advanced age do not rise 

 at night to a higher inclination than whilst young, yet they have 

 to pass through a larger angle (in one instance amounting to 

 63) to gain their nocturnal position, as they are generally 

 deflected beneath the horizon during the day. Even with the 

 11-days' old seedling the movement did not depend exclusively 

 on the pulvimis, for the blade where joined to the petiole was 

 curved upwards, and this must be attributed to unequal growth. 

 Therefore the periodic movements of the cotyledons of 0. corni- 

 culata depend on two distinct but conjoint actions, namely, the 

 expansion of the cells of the pulvinus and on the growth of 

 the upper part of the petiole, including the base of the blade. 



Lotus JacobcKiis. The seedlings of this plant present a case 

 parallel to that of Oxalis corniculata in some respects, and in 

 others unique, as far as we have seen. The cotyledons during 

 the first 4 or 5 days of their life do not exhibit any plain noc- 

 turnal movement ; but afterwards they stand vertically en* 

 almost vertically up at night. There is, however, some degree of 

 variability in this respect, apparently dependent on the season 

 anil on the degree to which they have been illuminated during 



