The modes of reproduction differ significantly between unicellular and multicellular organisms due to differences in their body design and cellular organization.

In unicellular organisms, reproduction is relatively simple. Since these organisms consist of a single cell, they can reproduce through simple methods like fission. New generations can be created from a single individual, which is known as asexual reproduction.

Multicellular organisms, however, cannot simply divide cell-by-cell. This is because they are not just a random collection of cells. Their specialised cells are organized as tissues, and tissues are organized into organs, which must be placed at definite positions in the body. In such a carefully organized situation, cell-by-cell division would be impractical.

Therefore, multicellular organisms need to use more complex ways of reproduction. A basic strategy is that different cell types perform different specialized functions. Reproduction in such organisms is the function of a specific cell type. There must be a single cell type in the organism that is capable of growing, proliferating, and making other cell types under the right circumstances.

Thus, the modes of reproduction depend on the body design of the organisms.