48 



In the Birches a.nd Alder the small winged nuts are produced on 3- 

 lobed bracts which are so arranged that they form a cone-like fruit- 

 ing body known as a strobile. It is rather hard to classify the fruits 

 of some species in terms of the types enumerated above, e. g., the 

 fruit of the Basswood has the appearance of a nut, but is in reality 

 a drupe; while the fruit of both the Mountain Ash and the Shad 

 Bush has the appearance of a berry but is actually a pome. A super- 

 ficial examination is often not sufficient to determine the type of 

 fruit. The fruit of our common Sumachs is a drupe, but is usually 

 covered with acid hairs, so that it is difficult to recognize the type 

 of fruit to which it belongs. 



After the fruits and seeds have been produced, it is necessary that 

 they be scattered on a mineral soil upon which they may germinate. 

 The distance over which they are scattered may be short or long, 

 depending upon the nature of the seeds and the agents by which they 

 are dispersed. The fruit, as a whole, is usually scattered in the case 

 of indehiscent fruits, while the seeds only are scattered in the case 

 of dehiscent fruits. The drawings on Plates IX and X show various 

 structural modifications of fruits and seeds which aid in their dis- 

 persal. The chief dispersal agents are propulsion, man, animals, 

 water, wind, and gravity. The Witch-hazel (Plate LXXXII), is a 

 good example of a species whose seeds are scattered by mechanical 

 propulsion. Man has been distributing seeds for forest trees inten- 

 tionally or unintentionally for many centuries, with the result that 

 the forest structure and landscape in many localities have been en- 

 tirely changed. Many European and Asiatic species have been 

 planted in America, and many of our native species like the Common 

 Locust and White Pine have a wide distribution abroad. Wind is 

 the most powerful of the dispersal agents. Many seeds have special 

 structural modifications which adapt them to be scattered by the 

 wind. The modifications may be a sac-like envelope (Plate IX, 10), 

 a mat of straight capillary hairs (Plate IX, 4, 6) or a membranous 

 winged, or flattened seed (Plate IX, 2 and Plate X, 3, 6, 11), Ani- 

 mals also scatter many seeds. A great number are scattered involun- 

 tarily by animals, especially such seeds as will hang fast to their 

 bodies. Other fruits are juicy and edible and are often eaten by 

 birds and other animals. A large number of our common birds swal- 

 low seeds to get the juicy edible portion surrounding them. These 

 seeds are not injured in passing through the alimentary canal of 

 birds, but in some cases it is thought that the seeds are even bene- 

 fited. The robins, thrushes, and blue birds eat a large quantity of 

 fleshy fruit and should be regarded as valuable agents for dispersing 

 seeds. The blue jay is also an agent that helps to scatter heavy 

 seeds like chestnuts and acorns. Other animals, especially rodents, 

 are also valuable as seed dispersal agents. Water, while not so 



