40 THE STORY OF THE PLANTS. 



similar round leaves, supported by a central leaf- 

 stalk, as is the case with the familiar garden 

 annual popularly (though erroneously) known as 

 nasturtium. (Its real name is Tropaeolum.) On 

 the other hand, when a plant has to struggle 

 hard for carbon and sunlight in overgrown 

 thickets, or under the water, it has usually very 

 much subdivided leaves, minutely cut, again and 

 again, into endless segments. Submerged leaves 

 invariably display this tendency. 



But that does not conclude the whole set of 

 circumstances which govern the forms and size 

 of leaves. Not only do they want to eat, and to 

 have access to sunshine ; they must also be sup- 

 ported or held in place so as to catch it. For 

 this purpose they have need of what we may 

 venture to describe as foliar architecture. This 

 architecture takes the form of ribs or beams of 

 harder material, which ramify through and raise 

 aloft the softer and actively living cell-stuff. 

 They are, as it were, the skeleton or framework 

 of the leaf; and in what are commonly known 

 as " skeleton leaves " the living cell-stuff between 

 has been rotted away, so as to display this harder 

 underlying skeleton or framework. It is com- 

 posed of specially hardened, lengthened, and 

 strengthened cells, and is intended, not only to 

 do certain living work in the plant (as we shall 

 see hereafter), but also to form a supporting 

 scaffolding. The material of which ribs or beams 

 are composed is called " vascular tissue " — a not 

 very well chosen name, as this material has only 

 a slight analogy to what is called the vascular 

 system (or network of blood-vessels) in an animal 

 body. It is much more like the bony skeleton. 



