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Show 408 MODIFIED CIHCUMNUTA'l'.ION. CHAr. VII. much in different species, and seems never to be rigid; for plants have been introduced from all pmts of the world into our gardens and greenhouses; and. if their movements 4acl been at all strictly fixed in relation to the alternations of clay and night, they would have slept in this country at very different hours, whieh is not the case. Moreover, it has been obscrv d that sleeping plants in their native homes chango their times of sleep with the changing seasons.* V'\T e may now turn to tho systematic list (p. 320). This contains the names of all the sleeping plants known to us, though tho list undoubtedly is very imperfect. It may be premised that, as a general rule, all tho species in the same genus sleep in nearly the same manner. But there arc some exceptions ; in several large genera including many sleeping species (for instance, Oxalis), some do not sleep. One species of Melilotus sleeps like a 'fri· folium, and therefore very differently from its con· geners ; so docs one species of Cassia. In the ge~us Sicla, the leaves either rise or fuJI at night; and With Lupinus they sleep in three different methods. R~turning to the list, the first point which strikes us, IS that there are many more genera amongst the ~egu· minosro (and in almost every one of tho Legummous tribes) than in all tho other families put together; anu we are tempted to connect this fact with the great ·attributing such movements to this cause than, for instauce, tho iuhoriled habit of winter and summer wheat to grow best at diflerent seasons; f(,r this habit is lost after o. few years, like the movements of loo.ves in darkness after a fow days. No doubt &orne effect must bo produc d on th.e seeds by the long-continued culh· vation of tho puront-plants un~~~ di1ferent climntc~. but 110 one P ~ bably would call tli~s the '' Nuc · wirlmnO'" of tho clun~t.tcs. * Pf~fier, ibid., P· 46. CuAP. VII. SUMMARY ON SLEEP OF LEAVES. 409 mobility of the stems and. leaves in this family, as shown by the large number of climbing species which it contains. Next to the Leguminosre come the Malvacere, together with some closely allied f<:tmilies. But by far the most important point in the list, is that we meet with sleeping plants in 28 families, in all tho great divisions of the Phaneroo-amic series, and in one Cryptogam. Now, although it is probable that with the Leguminosro the tendency to sleep may have been inherited from one or a few progenitors, and possibly so in the cohorts of the lVIal vaJ es and Chenopodiales, yet it is manifest that the tendency must have been acquired by the several genera in the other families, quite independently of one another. Hence the question naturally arises, how has this been possible ? and the answer, we cannot doubt, is that leaves owe their nyctitropic movements to their habit of circumnutating,- a habit common to all plants, and everywhere ready for any benc:ficial deYelopment or modification. It has been shown in the previous chapters that the leav?s and cotyledons of all plants are continually ~OVIng up and clown, generally to a slight but somet~ mes to a considerable extent, and that they describe etther one or several ellipses in tho course of twentyfou: hours; they are also so far affected by the alternatwns of clay and night that they generally, or at least often, move periodically to a small extent; and here we have a basis for the development of the greater nyctitropic movements. That the movements of I · . ~aves and cotyleclons which do not sleep come ~thm the class of circumnutating movements cannot h doubted, for they are closely similar to those of rpoc?tyls, epicotyls, the stems of mature plants, and of vanous other organs. Now, if we take tho simplest |