132 



LEAVES AND BUDS. 



examine a considerable number, and to this end the student should take 

 country rambles, supplied with a book for notes and sketches, and a 

 lens. The following instructions must be accepted as merely suggestive, 

 and by no means exhaustive. 



A typical leaf consists of a basal more or less sheathing portion, 

 called the leaf-sheath or leaf-base, a stalk or petiole, and a flattened 

 terminal portion, the blade or lamina. The leaf-sheath is usually 

 simply the flattened basal portion of the petiole, but in most grasses it 

 is of considerable size, and ensheathes the stem for some distance above 

 the node from which it arises. 



A sheathing leaf-base is frequently absent, but is especially well 

 developed in such plants as Buttercups, Grasses, Docks, Umbellifers. 

 The mode of insertion of different leaves exhibits a wide range of 

 variation, but they always arise from the shoot or its branches. 

 The so-called "radical" leaves simply arise from a very short 

 stem (Primrose, Dandelion). In some cases the base of the leaf 

 is decurrent, and forms membranous outgrowths on the main axis 

 (e.g. some Thistles), while it may even unite around the stem so as to 

 form a,perfoliate leaf (e.g. Yellow Gentian). 



Occasionally the leaf-base becomes thick and fleshy, and forms an 



irritable organ or organ of move- 

 ment known as the pulvinus, which 

 reacts to light in such a manner as 

 to place the lamina under the best 

 possible conditions of illumination. 

 This is possible owing to the fact 

 that the pulvinus retains the power 

 of growth for some time after that 

 of the rest of the leaf has ceased, as 

 can easily be observed by watching 

 Kidney Beans and altering the posi- 

 tion of the stem from time to time. 

 In some plants the pulvinus re- 

 sponds quite rapidly to changes in 

 the direction and intensity of the 

 light. Thus the compound leaf of 

 the Sensitive Plant (Fig. 46) places 

 itself at right angles to the light 

 falling upon it. Each leaflet, how- 

 ever, has a pulvinus of its own, 

 and when the leaf is exposed to 

 strong sunlight these pulvini curve 

 in such a manner as to cause the 

 leaflets to fold together in a few minutes, before they have time to be 

 injuriously affected. At night, also, the leaflets fold together and go 

 to sleep, as it were, while the whole leaf sinks downwards. In this 

 position the leaf loses less water by evaporation, and remains warmer 

 than it would be if expanded. The closing movements can also be 

 induced by contact, heat, and other stimuli. 



Fig. 46. Leaves of Mimosa pndica (the 



Sensitive Plant). 



a, Expanded day position ; b, Drooping 

 folded night position. 



