Chapter 19, Problem 34A

Summary Introduction

To analyze : 

The organism that is motile and has diploid nuclei but no separate cells.

Introduction :

Slime molds are fungus-like protists and belong to the Phylum Myxomycota. They do not possess chlorophyll and have phagotrophic or saprotrophic nutrition. They are of two types: acellular Slime molds and cellular Slime molds. The acellular smile molds are called so because they go through a free-living diploid somatic phase in their life cycle where a wall-less mass of multinucleate protoplasm covered by slime is formed.

Answer to Problem 34A

The correct term is plasmodium.

Explanation of Solution

The somatic phase of acellular slime molds is a diploid multinucleate plasmodium. Plasmodium is formed because cell wall formation is not followed by the division of the nucleus. All the nuclei in the plasmodium divide simultaneously.

The multinucleate protoplasm of the sporangium undergoes meiotic division to form uninucleate spores. The sporangia burst and release the spores. The spore falls on the suitable substratum and releases either biflagellate spindle-shaped swarm cell or non-flagellated myxamoeba. Myxamoebae fuse in pairs while swarm cells fuse in pairs at the posterior non-flagellate ends respectively, to form a diploid zygote. 

The zygote grows in size. The diploid nucleus of the zygote divides without the formation of cell walls. It changes into a multinucleate amoeboid structure known as plasmodium. The plasmodium, on maturity, bears sporangium, and the whole cycle repeats.

Conclusion

In the life cycle of acellular slime molds, the zygote forms plasmodium. It is a motile and multinucleate organism with no separate cells. It is because the nuclear division is not followed by wall formation or cytokinesis.