in a few genera, for instance, Trifolium and Geranium, which are included amongst the sleepers, such genera might therefore have been added to the 89.  Again, one species of Oxalis generally raised its cotyledons at night more than 20o and less than 60o 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, [page 113] cushion, or joint (as this organ has been variously called), like 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 whole of the short sub-petioles of the leaflets have been converted into pulvini.  With pulvinated leaves (i.e. those provided with a pulvinus) their periodical movements depend, according to Pfeffer,* on the cells of the pulvinus alternately expanding more quickly on one side than on the other; whereas the similar movements of leaves not provided with pulvini, depend on their growth being alternately more rapid on one side than on the other.** As long as a leaf provided with a pulvinus is young and continues to grow, its movement depends on both these causes combined;*** and if the view now held by many botanists be sound, namely, that growth is always preceded by the expansion of the growing cells, then the difference between the movements induced by the aid of pulvini and

Fig. 63.  Oxalis rosea:  longitudinal section of a pulvinus on the summit of the petiole of a cotyledon, drawn with the camera lucida, magnified 75 times:  p, p, petiole; f, fibro-vascular bundle:  b, b, commencement of blade of cotyledon.

* ‘Die Periodische Bewegungen der Blattorgane,’ 1875.

** Batalin, ‘Flora,’ Oct. 1st, 1873

*** Pfeffer, ibid. p. 5. [page 114]

without such aid, is reduced to the expansion of the cells not being followed by growth in the first case, and being so followed in the second case.