MORPHOLOGY 17 



known in the tropical, rock-inhabiting Discidia. In 

 this plant one leaf of a pair forms a bag, much like that 

 of a Pitcher plant, in which the adventitious roots from 

 each node are contained. 



Such extreme modifications are unusual, but every 

 normal plant has various kinds of leaves, and we must 

 now turn to the modified leaves which unite to form, 

 with all their infinite varieties, what we call the 

 flower. 



The essential parts of the flower are the sexual cells, 

 but, like the individual tissue cells, these are very minute, 

 and so, for their protection and assistance, a number 

 of leaves have become particularly modified on a given 

 plan which, in its essentials, is common to most flowers. 



The outer leaves of a flower are protective, and these 

 are generally green or brown and of strong texture. 

 In most of the higher plants they have a definite number, 

 often three, four, or five. Within them the next set 

 of leaves is generally more brilliantly coloured and of 

 more delicate texture. To this special series of leaves 

 the name corolla is given, and the individual leaves 

 are called the petals. Their work is entirely different 

 from that of ordinary leaves, and, while it is partly 

 protective, their use is largely to make the flower 

 attractive to the insects which come (or .used to come 

 in the past) to carry the pollen which effects cross 

 pollination. We next come to the more important 

 " leaves," which are reduced in general to small stalks, 

 bearing the male sporangia, called the pollen sacs. The 

 Sporangia belong to a distinct category of organ, and 

 though they arise on the modified (and in some families 

 on the normal) leaves, they are distinct from them in 

 just the same sense that the leaf is distinct from the 

 stem that bears it. Indeed the distinction is more 



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