522 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1908. 



out on the desert a few miles away and a mile lower down. Doubt- 

 less hundreds of thousands of seeds are carried to the lowlands each 

 year. Some of these develop into individuals which carry out repro- 

 duction. This is usually done in the native habitat, at actual tem- 

 peratures of the tissues not above 60° or 70° F. Down below spore 

 formation, reduction divisions, and fertilization may ensue in tem- 

 peratures 40° or 50° higher, a difference capable of being endured 

 by the shoots of some plants now being tested, and which might well 

 cause irreversible developmental changes. Other factors of the en- 

 vironment may operate in a similar manner. 



Again it is to be recalled that the actual formation or intrusion 

 of active substances in the ovarial tissues maj^ result from the stings 

 of insects, the mycelia of parasitic fungi, the penetration by foreign 

 pollen, or the egg or pollen may become subject to radium emanations 

 or to X rays or other forms of radiant energy. Still another possible 

 action is to be accounted for: in hybridization the foreign pollen 

 tubes, carrying the generative nuclei of the pollen parent, may en- 

 counter substances in the invaded pistil to which they are not usually 

 subject, with the result that its capacity for transmission of parental 

 characters may be altered, and qualities may thus appear in the 

 progeny which are not active in either parent. 



A hypothetical consideration of the known facts, as presented by 

 the many species in which mutation has been seen to occur seems to 

 lead to the conclusion that the changes upon which discontinuity of 

 inheritance rests ensue previous to the reduction divisions in plants. 

 The alterations which take place in my experiments, however, fol- 

 low disturbances not brought to bear upon the germ plasm until 

 after the second or third divisions following the reducing divisions, 

 and are perhaps separated from this act by a considerable period of 

 time. 



The induction of such new forms in plants may be accomplished 

 by reagents applied to the generative nuclei carried by the pollen 

 tube, and probably by action on the embryo sac, in the period fol- 

 lowing reduction division. Mutations have been taken, on hypo- 

 thetical grounds, to be based on changes occurring previous to these 

 divisions. 



A brief summary of the foregoing discussion may be made in the 

 following generalizations : 



1. Species may arise by hybridizations which result in fixed forms. 

 A large number of forms known as species, and recognized to be of 

 such origin, are known, and a number of them have been duplicated 

 in experimental cultures, some of which were made over a half cen- 

 tury ago. 



2. The mutation theory groups an enormous number of hitherto 

 unexplainable facts, to which we are constantly adding in great 



