Although the two plants are of the same species, hemp plants grown for fiber used to make rope are different from marijuana plants grown for flowers that produce THC (the "drug part") in many physiological and practical ways. As different as a wolf from a shih tzu, or a crabapple from a honeycrisp.

For the most part, THC is produced in the flower of the cannabis plant. Most cannabis plants are either male or female (not both), and only female plants produce flowers.

Since hemp plants are cultivated for fiber, they usually have thick, strong, stalks. It's better to grow them taller as opposed to wider, to fit more plants in a field. Both male and female plants can be used for fiber. Female hemp plants do grow small flowers, and those flowers do produce small amounts of THC, but not enough to be worth harvesting. Legally, modern hemp plants grown for fiber have less than one third of one percent THC content.

Since marijuana plants are cultivated for flowers, they usually have multiple, branching stalks, and they often spread and grow bushy at the top. It's better to grow them wider as opposed to taller, so each plant can spread out and produce multiple flower stalks. The thin, branching stalks of these relatively short female marijuana plants could be used for fiber, but there's probably not enough material there to be worth the effort. Meanwhile, many producers claim their marijuana flower to have 25% THC content or more.

It's thought that cannabis flowers produce THC for at least two reasons. One is that the compound is sticky and helps hold on to pollen that might drift past from nearby male plants. Another reason is that it acts as a sunscreen for the flowers. The flowers produce THC to capture pollen, and also to protect themselves from the sun when they are wide open and waiting for the pollen to come.

Cannabis seeds don't contain any THC (except whatever small amount may be left over from the flowers that produced them). All else being equal, the seeds of a hemp plant and the seeds of a marijuana plant should have the same value as a food source or industrial resource. Seeds from marijuana plants are rarer, though not necessarily more valuable.

One reason marijuana seeds are rare is that cannabis flowers produce way more THC when they are left unfertilized. The plant is producing THC in order to attract pollen, so as long as there is no pollen around, the plant just keeps producing more THC. It is by far most efficient to keep THC-producing female plants isolated from male plants. But this means those flowers are never fertilized and never produce any new seeds.

Long ago, it was common for marijuana bud to have seeds. Cannabis flowers grown outdoors are much more difficult to keep from being fertilized. Seedless marijuana bud, "sinsemilla," was an uncommon treat for many illicit cannabis consumers in the '70s, '80s, or even into the '90s. More recently, relaxed legal regulation and technological advances have made controlled indoor marijuana growing much easier and more effective, and much more common. These days the paradigm has flipped, and it's highly unusual (and maybe a little insulting) to find seeds in any flower purchased for THC consumption.