356 A TEXTBOOK OP BOTANY [Ch. VII, 3 



tinues to grow and (by action of digestive enzymes) absorbs 

 the endosperm, and even the nucellus, so that finally it comes 

 to fill completely the seed coats and contains all of the seed 

 food within itself. The shape of tlie emljryo is correlated 

 with that of the embryo sac, being short and straight in 

 some, variously elongated and curved or bent in others, often 

 to extreme degree. Whatever the shape, however, it fol- 

 lows from the mode of development of the embryo that 

 the root end of the hypocotyl lies next to the micropyle. 



3. The Dissemination and Dispersal of Plants 



Many features in fruits show obvious connection with the 

 dissemination of seeds, though fruits are by no means thus 

 completely explained. Pi'operly, dissemination means sim- 

 ply the scattering of seeds from the parent plant, but the 

 term is often employed more broadly, even to an equivalence 

 with dispersal, which means the spread of plants over 

 the earth's surface. Many parts besides fruits are here 

 concerned, so that we may best consider the subject as 

 a unit. 



That plants which have in themselves no power of free 

 locomotion such as animals possess can yet spread very 

 widely and quickly is shown by the familiar case of weeds, 

 which, introduced into a new territory, often overrun it 

 before man becomes aware of the danger. A striking pres- 

 ent-day instance is familiar in the deadly Chestnut disease, 

 a Fungus with wind-carried spores, which, introduced from 

 Asia into eastern America about ten years since, has already 

 spread through several states. Animals could hardly 

 spread faster. 



The physiological necessity for some method of dissem- 

 ination is amply obvious, for if all spores or seeds produced 

 by a plant were to germinate where formed, or on the ground 

 directly beneath, the resultant congestion would prevent 

 normal development of any of the plants. Green plants 

 need room to spread their foliage to the sun, and parasites 



