122 PULVINI OF COTYLEDONS. Chap. IL 



the day. With older seedlings, having cotyledons 4 mm. ic 

 length, which rise considerably at night, there is a well -deve- 

 loped pulvinus close to the blade, colourless, and rather nar- 

 rower than the rest of the petiole, from which it is abruptly 

 separated. It is formed of a mass of small cells of an average 

 length of -021 mm. ; whereas the cells in the lower part of the 

 petiole are about -06 mm., and those in the blade from -034 to 

 •04 mm. in length. The epidermic cells in the lower part of the 

 petiole project conically, and thus differ in shape from those 

 over the pulvinus. 



Turning now to very young seedlings, the cotyledojie of which 

 do not rise at night and are only from 2 to 2i mm. in length, 

 their petioles do not exhibit any defined zone of small cells, 

 destitute of chlorophyll and differing in shape exteriorly from 

 the lower ones. Nevertheless, the cells at the place where a 

 pulvinus will afterwards be developed are smaller (being on an 

 average 015 mm. in length) than those in the lower parts of 

 the same petiole, which gradually become larger in proceeding 

 downwards, the largest being -030 mm. in length. At this early 

 age the cells of the blade are about -027 mm. in length. We 

 tbus see that the pulvinus is formed by the cells in the upper- 

 most part of the petiole, continuing for only a short time to 

 increase in length, then being arrested in their growth, accom- 

 panied by the loss of their chlorophyll grains ; whilst the cells 

 in the lower part of the petiole continue for a long time to 

 increase in length, those of the epidermis becoming more conical. 

 The singular fact of the cotyledons of this plant not sleeping at 

 ' first is therefore due to the pulvinus not being developed at an 

 early age. 



We learn from these two cases of Lotus and Oxalis, 

 that the development of a pulvinus follows from the 

 growth of the cells over a small defined space of the 

 petiole being almost arrested at an early age. With 

 Lotus JoLcohseus the cells at first increase a little iii 

 length ; in Oxalis cornieulata they decrease a little, 

 owing to seli-division. A mass of such small cells 

 forming a pulvinus, might therefore be either acquired 

 or lost without any special difficulty, by different 

 species in the same natural genus : and we know that 



