If you will keep them all their life on 24/24...man, that's the vegetative program, they will grow and grow and grow, you will end up with a tree. Think that once you will consider in puting them to the flowering stage, the plant will double++ in size.
In the photosynthesis process itself, there is a reaction known as 'dark reaction' pathway or lately known as 'carbon reaction' pathway where the free energy of ATP and reducing power of NADPH, are used to fix and reduce CO2 to form carbohydrate. This is very important process to release Oxygen into the air. This happened in the dark or at night.
Secondly, for inducing the plant to initiate flowering process. This phenomena is known as photoperiod, ie based on the ratio of the daylength to night length over 24 hours. Plants reaction to photoperiod can be divided into 4 groups.
a. Day-neutral plants - the plants has no effect on photoperiod, They will flower when the plant reach physiological maturity.
b. Short day plants - plants will only flowering when daylength is shorter than nightlength. This group of plants will only flower in late summer or fall or very early spring, when day length is shorter than the night period. e.g. strawberries, potatoes, chrysanthemums etc.
c. Long day plants - Plant that flower in spring or early summer. eg lettuce, spinach, radish etc. Flower initiation will begin when the daylength is getting longer than the dark period.
d. Intermediate day plants - These group of plants will grow vegetatively when daylength is either too long or too short. They will only flower when the day length is about 12hrs of daylight and 12 hrs of darkness.
So...what type of weed are you growing?