Experimental Control of Zoid Discharge and Rhizoid Formation in the Green Alga Enteromorpha

Abstract

1. An isolate of Enteromorpha growing in the Shedd Aquarium, Chicago, was tested for zoid discharge under various environmental conditions, for the distribution of zoid-producing areas, and for polar and position effects of rhizoid production. 2. Discharge of zoids occurred almost predictably in 2-5 days in excised sections removed from the stock culture. 3. A wounding effect was tested by comparing discharge in similar lengths of thalli, half of which were cut and half left entire. No differences in discharge were noted. 4. Other changes encountered in transfer from stock culture to new media were discounted as factors inducing discharge. 5. Discharge in sections transferred to water from the stock culture lagged initially in comparison with discharge in sections transferred to fresh sea water, but final discharge indexes were equal. When the stock-culture water was autoclaved before plants were transferred into it, there was no lag period. This indicated that an unstable inhibitor was possibly present in crowded cultures. 6. When thalli were crowded into smaller volumes of water, discharge was less. 7. The effect of gross nutrient deficiency in the culture medium was tested by observing discharge in parallel graded concentration series of sea water and artificial sea water. Nutrient deficiency had no effect, while dilution reduced the discharge of zoids. 8. Addition of inorganic nutrients and soil solution enhanced discharge markedly. 9. No discharge response was evident in sections exposed to continuous light of less than 10 ft-c., or in sections exposed to 250 ft-c. for 1 hour or less per day. Above 100 ft-c. or 5 hours of light there was little change in response. 10. Discharge was affected by changes in pH, with greatest response in the range 6.5-7.5. 11. In sections discharging completely, two sequences of events were found: a leisurely emergence of zoids from scattered cells, or a fast, explosive discharge, beginning near the middle and spreading outward toward each end, with release of zoids only from the periphery of the expanding belt of empty cells. 12. Distribution of zoid-producing areas, both in random excised sections and in sections from known portions on branches, was more frequent at distal and proximal ends and much less in intermediate areas. 13. IAA supplied exogenously in the sea water (10-3-10-10 M) suppressed zoid liberation moderately at the higher concentrations, but there was no apparent effect at lower concentrations. 14. A single thallus, exposed simultaneously to two different culture media-a discharge stimulant and a discharge suppressor-released zoids only in the former. This was evidence against a mobile endogenous stimulant. 15. Polarity was noted in rhizoid production. Rhizoids formed only at the proximal ends of sections of Enteromorpha sp., E. clathrata, E. intestinalis, and E. linza. The distal end either remained open, closed up, or became swollen and lumpy. 16. In sections from known portions of thalli, rhizoids formed first, and most vigorously, from apical ones. There was a noticeable decrease in size in rhizoids of E. intestinalis from apical to basal sections, and also as the size of the section was decreased from 4.0 to 0.25 mm. in length.