Semilla vs. simiente

In Spanish, both 'semilla' and 'simiente' refer to the idea of a seed, but they are used in slightly different contexts.

Semilla

The word 'semilla' refers to the seed of a plant, that is, the reproductive unit that is capable of developing into another such plant. However, it is commonly used in everyday language and tends to be scientifically oriented towards botany.
Voy a plantar semillas de tomate en el jardín.
(I am going to plant tomato seeds in the garden.)
La semilla de manzana contiene cianuro, pero en cantidades muy pequeñas.
(The apple seed contains cyanide, but in very small amounts.)
Es importante seleccionar la mejor calidad de semillas para la agricultura.
(It’s important to select the best quality of seeds for agriculture.)

Simiente

Although 'simiente' can also mean a seed of a plant, its usage leans more towards literary or metaphorical contexts. It often carries connotations of origin, beginning, or potential for growth and development beyond botany.
La simiente de la discordia fue sembrada temprano en la relación.
(The seed of discord was sown early in the relationship.)
Abraham es considerado la simiente de muchas naciones.
(Abraham is considered the seed of many nations.)
De pequeñas simientes surgen grandes árboles.
(From small seeds grow great trees.)

Summary

While both 'semilla' and 'simiente' translate into English as 'seed' and can sometimes be used interchangeably, 'semilla' is more commonly used in everyday and scientific contexts to refer specifically to the physical seeds of plants. On the other hand, 'simiente' carries a broader, often metaphorical implication about origin and potential.