FRUITS 



55 



another example of a seed commonly called a nut. 

 We have to bear in mind the distinction between 

 true nuts and hard seeds. An example of the latter 

 is the horse-chestnut. Here the whole capsulelike 

 structure is the fruit and the "nuts" are hard- 

 coated seeds. On the other hand, the acorn cup is 

 made up of leaflike structures which together form 

 what is called an involucre. The chestnut bur is 

 another example oi the involucre which has become 

 prickly, the nuts being each a true fruit. 



Grain. — The grain, as we shall see when we 

 study the corn more carefully, is a fruit in which 

 the seed occupies most of the space within the fruit, 

 and the seed coat has become so closely attached to 

 the ovary wall that the two coats cannot be sepa- 

 rated. 



Key Fruit or Samara. — A very common inde- 

 hiscent fruit is found on the maple, ash, elm, and 

 other trees. It is the key fruit or samara. In this 



Grain ; spikes of ripened 

 flowers. 



case the pericarp has lengthened into a 

 long wing. 



Hold a maple or ash fruit high above 

 your head and allow it to fall to the ground. 

 Does it fall directly under the point where it 

 was held ? When and how might the wings 

 be of use to the fruit ? * 



Key fruit of maple. 



Distinction between Seeds and Fruits 



— We have seen that in the case of 

 one-seeded fruits it is sometimes 

 difficult to distinguish between the ^^^ 

 seed and the fruit. If we are able -^ 

 to examine the flower which forms 

 a certain fruit, we ought to have no 



difficulty in making out all of its parts. "^ ^^^Jpi)'^ 

 A cross section through the ovary will show 

 us that seeds are always surrounded not 

 only by seed coverings, but also by the ovary 

 wall, which later forms the pericarp. In 



' For laboratory exercises on dry indehiscent fruits, see 

 Hunter and Valentine, Manual, pages 69, 70, 74, 75. 



Chestnuts; 

 fruits sur- 

 rounded by 

 a prickly in- 

 volucre. 



