OCR Text |
Show 112 PULVINI OF COTYLEDONS. CHAP. It. cotyledons which at noon were nearly horizontal, stood at night at more than 20° and less than 60° above the horizon, they were recorded as " plainly raised ;" anu of such genera there were 38. We diu not moot with any distinct instances of cotyledons periodically sinking only a few degrees at night, although no doubt such occur. vVe have now accounted for 64 genera out of the 153, and there remain 89 in which the cotyledons did not change their position at night by as much as 20° -that is, in a conspicuous manner which could easily be detected by the unaided eye and by memory; but it must not be inferred from this statement that these cotyledons did not move at all, for in several cases a rise of a few degrees was re· corded, when they were carefully observed. The number 89 might have been a little increased, for the cotyledons remained almost horizontal at night in some species in a few genera, for instance, Trifolium and Geranium, which are incluued amongst the sleepers, such genera might therefore have been added to the 89. Again, one species of Oxalis generally raised its cotyleuons at night more than 20° and less than 60° above the horizon ; so that this genus might have been included under two heads. But as several species in the same genus were not often observed, such double entries have been avoided. In a future chapter it will be shown that the leaves of many plants which do not sleep, rise a few degrees in the evening and during the early part of the night; and it will be convenient to defer until then the consideration of the periodicity of the movements of cotyledons. On the Pulvini or Joints of Cotyledons.-With several of the seedlings described in this and the last chapter, the summit of the petiole is developed into a pulvinus, CIIAP. II. PULVINI OF COTYLEDONS. 113 cushion,. or joint ~as th~s organ has been variously called), hke that with whiCh many leaves are provided. It consists of a mass of small cells usually of a pale colour from the absence of chlorophyll, and with its outline more or less convex, as shown in the annexed figure. In the case of Oxalis sensitiva two-thirds of the petiole, and in that of Mimosa pudica, apparently the 1J whole of the short subpetioles of the leaflets have been converted in to pul vini. With pulvinated leaves (i.e. t~ose provided with a pulvmus) their periodical movements depend, according to Pfeffer,* on the cells of the ?ulvinus alternately expand-mg more quickly on one side than on the other ; whereas Fig. 63. b Oxalis t·osea: longitudinal section of a pulvinus on the summit of the P.etiole of a cotylet!ou, drawn w1th the camera lucitln ~agnifi.ed 75 times: p, p, pe~ bole;/, fibro-vascular bundle; b, b, commencement of blade of cotyledon. the similar movements of l~a:es not provided with pulVI~ I, depend on their growth bemg alternately more rapid on one side than on the other.t As long as a leaf provided with a pulvinus is roung and continues to grow, ~~:.oveme~t depends on both these causes combined ;:j: If the VIew now held by many botanists be sound ~amely, that growth is always preceded by the expan~ tshw n of the g row·m g ce II s, t h en the di.f ference betwe' en e movements induced by the aid of pulvini and • 'Die Periodische Beweo-ungen der Blattorgane,' 1H75. 0 t B11:talin, 'Flora,' Oct.Ist, 187B. t Pfeffer, ibid. p. 5. I |