IX Leaves 185 



the plants were grown in a very damp atmosphere, and 

 (2) that intense illumination increases spininess, while 

 restricted illumination tends to suppress it. 



In this connection may be read with advantage the plea 

 for experiment in Dr. de Varigny's recent Edinburgh lec- 

 tures {Experimental Evolutio?t^ Nature Series, 1892). 



Very important as protective structures are the bud- 

 scales, which are often hard and varnished, and effectively 

 shield the tender leaves of the bud from frost and damp. 

 We have already spoken of their importance, and have 

 noticed that they are modified leaves or leaf- stalks, or in 

 some cases stipules. Thus on the horse-chestnut you may 

 find many gradations between the brown, sticky scale and 

 the leaf- stalk with its five leaflets. Some have simply a 

 little tip of green, in others this green tip is notched, in 

 others it is definitely divided into five lobes. 



Another special function discharged by many modified 

 leaves is that of storing. Sometimes they are swollen with 

 water, as in the Sedums and Aloes. Oftener they are 

 receptacles for starch and other products which the plants 

 manufacture. Thus the leaf-bases of the primrose which 

 persist after the leaves die away are full of starch ; and 

 the crowded, much modified leaves which form the greater 

 part of the bulb of an onion or a hyacinth are also store- 

 houses, from which in spring materials for new growth are 

 absorbed. 



Some leaves are so brightly coloured that when they 

 occur near the flower they may be mistaken for petals. 

 We see this very well in a common greenhouse plant, 

 the "Christmas flower" of Mexico, Poinsettia {P. pul- 

 cherrimd) ; and the great white spathe which surrounds 

 he flower of the Ethiopian "lily of the Nile" {Richardia 

 {Calld) africa7ta) is also a modified leaf. 



Lastly, as every one more or less clearly knows, at least 



