The first sign of flowering in Cannabis is the appearance of undifferentiated flower primordia along the main stem at the nodes (intersections) of the petiole, behind the stipule (leaf spur). In the prefloral phase, the sexes of Cannabis are indistinguishable except for general trends in shape.
When the primordia first appear they are undifferentiated sexually, but soon the males can be identified by their curved claw shape, soon followed by the differentiation of round pointed flower buds having five radial segments. The females are recognized by the enlargement of a symmetrical tubular calyx (floral sheath). They are easier to recognize at a young age than male primordia. The first female calyxes tend to lack paired pistils (pollen-catching appendages) though initial male flowers often mature and shed viable pollen. In some individuals, especially hybrids, small non-flowering limbs will form at the nodes and are often confused with male primordia.
Cultivators wait until actual flowers form to positively determine the sex of Cannabis
The female plants tend to be shorter and have more branches than the male. Female plants are leafy to the top with many leaves surrounding the flowers, while male plants have fewer leaves near the top with few if any leaves along the extended flowering limbs.
*The term pistil has developed a special meaning with respect to Cannabis which differs slightly from the precise botanical definition. This has come about mainly from the large number of cultivators who have casual knowledge of plant anatomy but an intense interest in the reproduction of Cannabis. The precise definition of pistil refers to the combination of ovary, style and stigma. In the more informal usage, pistil refers to the fused style and stigma. The informal sense is used throughout the book since it has become common practice among Cannabis cultivators.
The female flowers appear as two long white, yellow, or pink pistils protruding from the fold of a very thin membranous calyx. The calyx is covered with resin exuding glandular trichomes (hairs). Pistillate flowers are borne in pairs at the nodes one on each side of the petiole behind the stipule of bracts (reduced leaves) which conceal the flowers. The calyx measures 2 to 6 millimeters in length and is closely applied to, and completely contains, the ovary.
In male flowers, five petals (approximately 5 millimeters, or 3/16 inch, long) make up the calyx and may be yellow, white, or green in color. They hang down, and five stamens (approximately 5 millimeters long) emerge, consisting of slender anthers (pollen sacs), splitting upwards from the tip and suspended on thin filaments. The exterior surface of the staminate calyx is covered with non-glandular trichomes. The pollen grains are nearly spherical slightly yellow, and 25 to 30 microns (p) in diameter. The surface is smooth and exhibits 2 to 4 germ pores.
Before the start of flowering, the phyllotaxy (leaf arrangement) reverses and the number of leaflets per leaf decreases until a small single leaflet appears below each pair of calyxes. The phyllotaxy also changes from decussate (opposite) to alternate (staggered) and usually remains alternate throughout the floral stages regardless of sexual type.
The differences in flowering patterns of male and female plants are expressed in many ways. Soon after dehiscence (pollen shedding) the staminate plant dies, while the pistillate plant may mature up to five months after viable flowers are formed if little or no fertilization occurs. Compared with pistillate plants, staminate plants show a more rapid increase in height and a more rapid decrease in leaf size to the bracts which accompany the flowers. Staminate plants tend to flower up to one month earlier than pistillate plants; however, pistillate plants often differentiate primordia one to two weeks before staminate plants.