WOOD SORREL 129 



look of wood sorrel tells us how unfit it is to face rude 

 extremes. It is pale and fragile, with tender green leaves, 

 slight flower-stalks, and petals delicately veined with 

 purple. In one particular only does wood sorrel show 

 an unexpected hardiness. Provided that it can get 

 adequate shelter, it will endure arctic cold. It does not 

 die down in winter, and endures frost without even closing 

 its leaflets. In April not a few wood sorrel leaves, which 

 have lasted the winter through, are still green, and still 

 open and close, while all the old clover leaves are dead. 



Wood sorrel leaves close, not only by night, but when- 

 ever the sun shines full upon them, whenever they are 

 beaten by rain, or much blown upon, or rudely touched 

 by moving objects. This sensitiveness would almost 

 hinder them from assimilating enough to keep the plant 

 alive, if the assimilating surface were to be concealed 

 whenever they drooped. As it is, the leaves, even when 

 closed, can profit by a weak horizontal light, the kind of 

 light which best suits a shade-loving plant. 



The long leaf-stalk renders the mechanism of erection 

 and depression more effective, and makes it easier to bring 

 the leaflets into the most favourable light-position. Wood 

 sorrel is most at home where the shade of the trees is broken 

 by frequent patches of sunlight ; in such situations the 

 power of moving the leaf an inch this way or that may be 

 of great value. The plane of the expanded leaf can be 

 inclined, so as to catch the light better. Wood-ruff too, 

 a plant subject to similar conditions, is enabled in a 

 different, but not less effectual way, to set its leaf-planes 

 as nearly as possible at right angles to the rays of light. 

 An organ of movement is a necessary part of the equip- 

 ment of the wood sorrel leaf. Look at the enlarged base 

 of the leaf-stalk, and you will find a thin ring of tissue 

 distinct to the eye, especially when, as is often the case, 

 the stalk is reddish and the ring pale-green (Fig. 34 b). The 

 chief use of the ring in effecting leaf-movements is best 

 seen when wood sorrel growing in a garden is examined 



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