This photo shows a longitudinal dissection of Catharanthus roseus. The flower is in salverform, which means petals are fused to form a floral tube. Its calyx consists of 5 sepals and its corolla consists of 5 petals. Stamens are positioned between petals. It has 5 epipetalous stamens, which means stamens are attached to the corolla. Despite having 1 pistil and 1 style, there are 2 separate ovaries inside the pistil. The ovaries are superior.

Catharanthus roseus originate from Madagascar, now has been widely cultivated for ornamental and medicinal purpose. Many vinca alkaloids, which have been used for treating blood cancer, were discovered from Catharanthus roseus. For example, vinblastine and vincristine are used in treatment against leukemia and Hodgkin's lymphoma. But Catharanthus roseus is toxic if we consumed orally.

I would like to know what makes it toxic and what are the effects if we consume it.

Reference: https://web.archive.org/web/20070927032628/http://www.drugdigest.org/DD/PrintablePages/herbMonograph/0%2C11475%2C4108%2C00.html