2 90 TEXTBOOK OF BOTANY 



are ripe. Their seeds become free only when the fruit is 

 eaten or broken open, or when its coat decays ; or, as is the 

 case with the fruits of the corn, the thistle, and the oak, the 

 seed germinates within the fruit coat, which, like the seed 

 coat, is broken by the growth of the embryo. 



293. Nature and Importance of Seeds. — We know that 

 the seed contains a small plant (the embryo) whose growth 

 has been stopped for a time. We know, too, that the embryo 

 can begin to grow again when conditions become favorable ; 

 and that the seed also contains a good deal of food that the 

 young plant may use in its growth. The habit of stopping 

 the growth of the embryo at a certain point, and of shutting 

 it up with a supply of food inside the seed coat, marks one 

 great difference between the seed plants and all other plants. 

 This habit is of great advantage, because the young plant so 

 protected may be carried in one way or another to a long 

 distance from the parent, may remain unchanged until condi- 

 tions about it are favorable, and may then grow rapidly, using 

 the food that was stored in the seed. 



Just as different seed plants bear very different sorts of 

 flowers and fruits, so they produce many different kinds of 

 seeds. Every one knows that seeds vary greatly in size. 

 In some plants (some orchids, for example) the embryo is still 

 very small when the seed is ripe, sometimes consisting of only 

 a few cells ; in others (such as the bean and the Indian com) 

 it has a small root and stem (the radicle) , a terminal bud (the 

 plumule) , and several leaves. Often, as in the com, the food 

 is nearly all stored in the endosperm ; but some seeds, like 

 those' of the squash and the bean, contain little or no endo- 

 sperm when they are ripe, and most of their food is stored in 

 the seed leaves. Most seeds have a hard, tough seed coat ; 

 in the horse-chestnut and the Brazil nut the hard seed coat 

 seems much like the fruit coat of a true nut, such as the walnut 

 or acorn. Some seeds, on the other hand, like those ofthe 

 true nuts and of stone fmits, have soft, thin seed coats ; but 



