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Show 396 1\IODIFIED CIRCUMNUTATION. CHAl' . VII. petioles are. effected in two different ways ; firstly, by alternately mcreased growth on their opposite sides preceded by increased turgescence of the cells; ancl secondly by means of a pulvinus or aggregate of small cells, generally destitute of chlorophyll, which become alternately more turgescent on nearly opposite sides; and this turgescence is not followed by growth except during the early age of the plant. A pulvinus seems to be formed (as formerly shown) l>y a group of cells ceasing to grow at a very early age, and therefore does not differ essentially from the surrounding tissues. The cotyledons of some species of Trifolium are provided with a pulvinus, and others are destitute of one, and so it is with the leaves in the genus Sida. We see also in this same genus gradations in tho state of the development of the pulvinus; and. in Nicotiana we have what may probably be considered as tho commencing development of one. The nature of tho movement is closely similar, whether a pnlvinus is absent or present, as is eviclent from many of tho diagrams given in this chapter. It deserves notice that when a pulvinus is present, the ascending and descending lines hsudly ever coincide, so that ellipses are habitually described by the leaves thns provided, whether they are young or so old as to have quito ceased growing. This fact of ellipses being described, shows that the alternately increased. turgescence of the cells does not occur on exactly opposite sides of the pulvinus, any more than the increased growth which causes the movements of leaves not furnished with pulvini. When a pulvinus is present, the nyctitropic movements are continued for a very much longer period than when such clo not exist. This has been amply proved in the case of coty led.ons, and Pfeffer has given observations to the same effect with respect CuAP. VII. SUMMARY ON SLEEP OF LEAVES. 397 to leaves. vVe have seen that a leaf of Mimosa pudica continued to move in the ordinary manner, though somewhat more simply, until it withered and died. It may be added that some leaflets of Trifolium pratense were pinned open during 10 days, and on the first evening after being released they rose up and slept in the usual manner. Besides the long continuance of the movements when effected by the aid of a pulvinus (and this appears to be the final cause of its development), a twisting movement at night, as Pfeffer has remarked, is almost confined to leaves thus provided. It is a very general rule that the first true leaf though it may differ somewhat in shape from th~ leaves on the mature plant, yet sleeps like them ; and this occurs quite independently of the fact whether or not th.e cotyledons themselves sleep, or whether they sleep m the same manner. But with Phaseolus Roxburghii the first unifoliate leaves rise at night almost sufficiently to be said to sleep, whilst the leaflets of the secondary trifoliate leaves sink vertically at night. ?n yo~ng ~lants of Sida rhornbrefolia, only a few mches m height, the leaves did not sleep, though on rather older plants they rose up vertically at night. On :he other hand, the leaves on very young plants of Oyt'lBUB fragrans slept in a conspieuous manner whilst on 0l d an d V·I gorous bushes kept in the green' house the leaves did not exhibit any plain nyctitropic move~ ~ent. ~n the genus Lotus the basal stipule-like ~afiets rise up vertically at night, and are provided With pulvini. th~s already remarked, when leaves or leaflets chano-e eu ·r . o posi IOn greatly at mght and by complicated :o~ements, it can hardly be doubted that these must 10 some manner beneficial to the plant. If so, we |