Spores and seeds are both reproductive units, but they differ significantly in their structure and function. Here's a breakdown of their key differences:

Spores:

* Structure: Spores are single-celled, microscopic structures with a tough outer wall (often containing sporopollenin, a very resistant polymer). They contain a small amount of cytoplasm and a nucleus, but lack an embryo.

* Function: Spores are typically dispersed by wind or water and can survive harsh conditions for long periods. They germinate to produce a haploid gametophyte, which then produces gametes for sexual reproduction.

* Organisms: Spores are the primary means of reproduction in many plants, including ferns, mosses, and algae. They are also found in some fungi and bacteria.

Seeds:

* Structure: Seeds are multicellular structures containing an embryo, a food source (endosperm), and a protective seed coat. The embryo is a multicellular, miniature plant with a root, stem, and leaves.

* Function: Seeds are dispersed by various means (wind, animals, water) and can germinate under suitable conditions. The embryo grows into a new plant, and the endosperm provides it with nutrients until it can photosynthesize.

* Organisms: Seeds are characteristic of flowering plants (angiosperms) and gymnosperms (conifers, cycads, etc.).

Here's a table summarizing the key differences:

| Feature | Spores | Seeds |

|---------------|---------------------------------|-------------------------------------------|

| Structure | Single-celled, microscopic | Multicellular, macroscopic |

| | No embryo | Embryo present |

| | Tough outer wall | Seed coat |

| Function | Germinate into a gametophyte | Germinate into a new sporophyte |

| | Dispersed by wind or water | Dispersed by various means |

| Organisms | Ferns, mosses, algae, fungi, bacteria | Flowering plants (angiosperms) and gymnosperms |

In short, the key differences are:

* Complexity: Seeds are more complex structures than spores.

* Embryo: Seeds contain a multicellular embryo, while spores do not.

* Food source: Seeds have a food source (endosperm) to support the embryo's growth.

* Reproduction: Spores are involved in asexual or haploid reproduction, while seeds are involved in sexual and diploid reproduction.