33^ MORPHOLOG y. 



vascular system for conduction of water and food. In the gymnospcrms 

 and angi(;sf)erms the thallus is jiresent in the t-i^dospcnn; distinct, though 

 reduced, archegonia are present in most gymnosperms and represented 

 only by the egg in the angiosperms; the vascular system is still more highly 

 developed while the seed f(jr the first time is organized, and characterizes 

 these plants so that they are called seed plants, or Spcnnatophytes. 



Variation, Hybridization, Mutation. 



674a. Variation. — It is a well-known fact that plants as well as ani- 

 mals are subject to variation. L'nder certain conditions, some of which 

 are partly understood and others are unkncm-n, the progeny of plants dif- 

 fer in one or more characters irom their jjarcnts. Some of these variations 

 are believed to be due to the inllucnce of environment (sec Parts III and 

 IV). Others are the result of the crossing of individuals which show 

 greater or lesser differences in one or more characters, or the crossing of 

 different species (hybridization). The most profound variations are those 

 which spring suddenly into existence (mutation). 



674b. Hybridization. — Two different species are "crossed" where the 

 egg-cell of one species is fertilized by the sperm of another species. The 

 progeny resulting from such a cross is a Jiyhrid. Hvbrids sometimes resem- 

 ble one parent, sometimes another, sometimes both. Where the parents 

 differ only in respect to one character of an organ or structure, there is a 

 regular law in resi)ect to the progeny if they are self-fertilized. In the 

 hrst generation all the individuals are alike and resemble one of the parents, 

 and the special differential character of that parent is called the dominant 

 character. In the second generation 75% possess the dominant charaiter, 

 while 25% resemble the other original parent, and its differential charac- 

 ter is called recessive. These are pure recessives, since successive genera- 

 tions, if self-fertilized, are always recessive. (")f the y^'^'e ^'^'hich show the 

 dominant character in the second generation, one-third (or :?5'7' '">f the 

 whole numljer) are pure dominants if self-fertilization is continued, while 

 <,o^]r are really "cross brcrls" like the first generation, and if self-fertilized 

 split up again into approximately 25 dominants, 50 cross breds, and 2=; 

 recessives. This is what is called Mendel's law. Where the original par- 

 ents differ in resjject to more than one character, the result is more compli- 

 cated ("see Mendel's Prind[)les of Heredity; also de \'ries. Das Spaltungs- 

 gesetz der ISastarde, I'er. d. deutsch. bol. (lesell., 18, S:^, n)oo). 



674c. Mutation. — This term is applied t<> th(^se variati-ms which appear 

 so suddenly that .some of the progen_\' of two like indi\-iduals differ from all 

 the others I0 a marked degree. Some of these mutations are so different 

 as to be regarded as new species. Soiue ^il the primroses show mutations, 

 and ("Enothera gigas is a mutaliun fnim 'Tn(.)thera lamarkiana (see de \'ries. 

 Uie Mutalionstheorie, Leipzig) 



