402 Morpho genetic Factors 



In the male mycelium antheridial branches are first induced by hormone 

 A, produced by the female mycelium. The antheridial branches then 

 form hormone B, which induces the production of oogonial initials in 

 the female plant. These structures now form hormone C, which causes 

 the antheridial branches to grow chemotropically toward the oogo- 

 nial initials. Lastly, it appears that hormone D, presumably formed 

 in the antheridia, causes the oogonial initials to delimit the oogonia 

 from their stalks. The chemical nature of these substances is still un- 

 known. 



The most complex examples of the effects of specific substances in 

 sexual reproduction and sex determination in the lower plants are those 

 described by Moewus ( 1940 and many other papers ) in the unicellular 

 green alga Chlamydomonas eugametos, in which the biochemical and 

 the genetic basis of the various hormonal mechanisms were subjected to 

 detailed analysis. A thorough reexamination of this work indicates that 

 many of the facts and conclusions of Moewus are not well founded and 

 that the contributions of the Chlamydomonas work to our understand- 

 ing of sexuality in the lower plants are much less considerable than 

 they were once thought to be. 



Work on the sexual processes and substances in thallophytes has been 

 reviewed by J. Raper (1952, 1957). Hawker (1957) has reviewed the 

 whole field of reproduction in the fungi. 



Wound Hormones. The substances first proved to have a determining 

 effect on morphogenetic processes were the wound hormones, or necro- 

 hormones. It has long been observed that in the vicinity of dying and 

 necrotic cells there occur divisions in other cells which under ordinary 

 conditions would not have shown such division. These have a definite 

 relation, both in distribution and orientation, to the accumulation of de- 

 composition products released by the injured cells. Wound meristems are 

 thus developed which produce layers of cork that cut off the injured 

 region and protect the healthy tissue underneath. 



Haberlandt (1921, 1922) was the first to attack this problem directly. 

 He found that if the cut surface of a potato tuber is washed and the 

 contents of the injured cells thus removed only a few cell divisions 

 occur. It might be thought that in such cases the reduced access of 

 oxygen to the flooded tissues would account for the reduction in 

 metabolic activity and thus of cell division. The action of a definite 

 substance, however, is strongly indicated by later experiments of 

 Haberlandt and others in which the juice, debris, or extracts of tissues 

 produced an effect on cell division much exceeding that from mere 

 wounding (Fig. 18-22). When sap from crushed tissue was injected into 

 small intercellular spaces, active cell division, presumably from wound 

 hormones, was found to occur (Reiche, 1924). These substances are not 



