388 PLANT PHYSIOLOGY 



The Seed. — The seed is the chief organ of dispersal rather than 

 of reproduction, in the proper sense of the term, but it may be 

 considered briefly at this point. A reproductive organ is one 

 which contains sex cells and, although the ovule contains a female 

 gametophyte with an egg cell, the seed or ripened ovule contains 

 no such reproductive cells. It contains, on the other hand, an 

 embryo plant which it maintains during conditions unfavorable 

 for growth and then supplies with food when the germination 

 season arrives. Stamens and pistils have also been called repro- 

 ductive organs but, for similar reasons, the term is a misnomer 

 when applied to these structures. As the science of botany de- 

 veloped, it was once thought that the pollen grains were male 

 sex cells and the ovules were eggs, but the results of morphologi- 

 cal research have made such views absurd. Names, however, 

 do not change as rapidly as ideas. 



The dissemination of the seed is brought about by water, wind, 

 animals, and mechanical contrivances of the fruit or seed. The 

 coconut is especially equipped for long sea voyages over salt 

 water, and nuts from the West Indies have been carried by the 

 Gulf Stream as far as Norway without being impaired in their 

 viability. 



Wind-scattered seeds generally have hairy appendages as in 

 the willow (Salix), milkweed (Asclepias), and dandelion (Tarax- 

 acum), or are equipped with wings as in the elm (Ulmus), maple 

 (Acer), and linden (Tilia). 



Animal distribution is commonly accomplished through the 

 possession of edible and attractive fruits as in the apple, cherry, 

 and strawberry, all of which contain seeds that are thrown away 

 or are capable of resisting the juices of the digestive tract. An- 

 other method is by means of mucilaginous secretions or prongs 

 which catch in the clothes and fur of passing animals. The mistle- 

 toe (Phoradendron) has sticky seeds, and the beggar-lice (Bidens) 

 and puncture weed ( Tribulus) are excellent examples of the 

 latter type of dissemination. In this case, it is more often the 

 fruit than the seed which is provided with the adhering pro- 

 jections. 



Many types of mechanical contrivances are used by plants for 

 scattering the seeds. In touch-me-not (Impatiens) a slight pres- 

 sure on the ripened ovary wall causes the pod to snap open and 

 scatter the seeds. In the melon (Cucurbitacese) are to be found 



