Few seeds produce the allure of the castor bean. Each seed has a genetically distinct "face," a pattern unique as a fingerprint to that particular seed, according to Palomar College's Wayne's Word website, giving the seeds enormous visual appeal, in addition to their usefulness in studying seed structures. At the same time, at 12,000 times more poisonous than rattlesnake venom, castor beans are among the most toxic plants on Earth.
Despite their attractive appearance and usefulness in illustrating certain aspects of seed structure, castor beans contain the chemical ricin, one of the most toxic chemicals found naturally. The pretty, boldly patterned seeds particularly appeal to children, and ingesting just one seed can kill a child, according to the Cornell University Animal Science website. When exploring the structure of this intriguing seed, use extra care in handling actual beans.
Castor bean seeds possess several features that define their relationship to other plants. Castor seeds are endospermic, meaning that each seed contains a nutrient reserve. The embryonic plant inside is called a dicot, named for its two primitive leaves, one of two major groupings of flowering plants.
The tough and brightly patterned outer coating of the castor bean is called the testa, or seed coat. The seed coat provides protection to the plant embryo inside, guarding it from damage and drying out. To the person who accidentally swallows a castor bean, the seed coat also serves a protective function. As Dr. Leubner of the Seed Biology Place points out, the seed coat must break in order for the toxic ricin it contains to be absorbed by the intestine.
The thick endosperm encases the plant embryo and provides its nutritional needs until the plant emerges from the soil and begins the process of photosynthesis. The embryonic leaves of the castor bean feed on the endosperm, the cells of which undergo a programmed death as they release nutritional contents for the embryo to use. The endosperm contents provide the nutrients the plants need to break free from the seed.
At the end of each seed, you will find a small, fleshy structure called the caruncle, described on Wayne's Word as resembling the head of a tick attached to an engorged body. The caruncle helps the seed to absorb the water the embryo needs to grow. The caruncle also appeals to ants, which grab a hold of it and help to carry the seed to new locations, far away from the parent plant.