ALTERNATION OF GENERATIONS. 



203 



where the Uredo-form is produced from the -^cidium-form, and reproduces itself 

 through several generations, till at length from the last Uredo a Puccinia 

 springs as the third form of generation, and this, on its part, again produces an 

 ^Ecidium. (Cf Book II. Fungi.) The more common case among Algse and Fungi, 

 and the one exclusively met with among Muscineae and Vascular Cryptogams is, 

 however, for only two kinds of generations to alternate Avith one another, the 

 form A producing B, B again producing A, and so on. 



The whole process of development which passes through the successive 

 dissimilar generations, and finally returns again to the first form, is called Alter- 

 nation of Generations ; each form of generation which follows and precedes a dis- 

 similar one may be termed an Alternate Generation. The alternation of generations 

 A B, A B, A B, &c., for example, consists thus of the two alternate generations 

 A and B, and in like manner the alternation of generations A B B B &c. C, 

 A B B B &c. C of the three alternate generations A, B, and C. 



The reproductive cells either continue their development independently without 

 foreign aid, and are then termed Asexual Reproductive Cells, and the generation 

 from which they are directly and solely derived is called an Asexual Generation; 

 or they are so constituted that they do not attain a condition in which they 

 can develope without further union with another reproductive cell ; in this case 

 the reproductive cells are termed Sexual, and the form of generation to which 

 they owe their origin a Sexual Generation. If the two sexual cells which unite 

 to produce one capable of development are externally similar in form and size, 

 the union is termed Cojijugation ; if, on the other hand, they are conspicuously 

 dissimilar in form, size, and other respects, the union is termed Fertilisation. 

 That one of the two sexual cells which performs active work and itself dis- 

 appears after fertilisation is the j\[ak or Sperm-cell (spermatozoid, antherozoid, 

 pollen-grain) ; the one which is acted on by the former and becomes transformed 

 into an Embryo which begins the new generation is the Female or Germ-cell (ovum, 

 oosphere, germinal-vesicle) ^ While the asexual reproductive cells usually become 

 detached from the mother-plant and dispersed (hence called spores), in order to 

 produce the new generation at a distance from it, the germ-cell, on the contrary, 

 generally continues to lie in a special organ of the mother-plant (the oogonium, 

 archegonium, ovule), there awaits fertilisation, and, afterwards still nourished 

 by the mother -plant, commences the new process of vegetation (formation of 

 embryo). It does, however, occur also in Algae {e. g. in Fucaceae) that the germ-cells 

 escape before fertilisation, and produce the new generation without the help of 

 the mother-plant. 



If the relationship of asexual and sexual reproduction to the succession of 

 generations is observed, it is seen that in the most simple plants asexual genera- 

 tions sometimes follow one another without intermission, as, e. g. in Nostocaceae ; 

 while in other cases an uninterrupted series of sexual generations may succeed 

 one another, as in Spirogyra. When alternation of generations occurs, in certain 

 cases all the alternate generations may be asexual, as in Hydrodictyon (according 



Compare Book III, on Sexuality. 



