374 PLANT PHYSIOLOGY 



characteristics of reproduction, it is not an essential one. A poplar 

 tree may produce annually twenty-five million seeds, of which a 

 few survive. The fungi produce millions of spores, all capable of 

 forming new plants if placed under a favorable environment. In 

 these cases, there is a pronounced multiplication of individuals, 

 but in many cases the number of new individuals produced is not 

 sufficient to replace the two parents when they die. 



Although cytology and comparative morphology have taken 

 over the fields of botany which deal with the formation of the 

 sex cells and the way in which various plants have solved the 

 problem of reproduction, a brief resume of the physiological as- 

 pects of the subject is here included for those not familiar with 

 these fields. 



Asexual Reproduction. — Reproduction is of two types depend- 

 ing upon the number of parents required to complete the process 

 and to form the new individuals. In asexual reproduction, only 

 one parent is needed. No special sex cells are formed and, for these 

 reasons, this type of reproduction is also called vegetative, agamic, 

 and monogenetic. Although this is the simplest type of reproduc- 

 tion and is the only kind found among the earliest of plants such 

 as the bacteria and the blue-green algae, in which the mature cell 

 when ready to reproduce simply splits in two forming two cells 

 where one was before, it also occurs in the highest plants along 

 with sexual reproduction. This simplest type of asexual reproduc- 

 tion, called fission, also is found among the lowest animals. 



Many of the simpler plants form special reproductive bodies 

 which are usually one-celled and capable of resisting dry weather, 

 cold, etc., for some time. These bodies, called spores, are formed 

 in great numbers by the Fungi, Bryophytes, and Pteridophytes. 



In addition to spores, the Bryophytes (Liverworts) develop 

 specialized groups of cells called gemmse which are capable of re- 

 producing the entire thallus of the plant. Similarly in the seed 

 plants, many forms have special asexual methods of reproduction 

 in which some vegetative tissue is especially fitted for reproducing 

 the plant. Among these may be mentioned the bulbs of the onion, 

 the tubers of the potato and artichoke, the corms of Jack-in-the- 

 pulpit (Arissema), the runners of the strawberry (Fragaria), etc. 

 In fact all the " reproductive stems" are examples of asexual repro- 

 duction. Roots also may serve this same purpose, as in the dahlia 

 and sweet potato. Many of our most troublesome weeds, like 



