So why length? Why not dry mass?
The answer is simple: I'm starting with cuttings, not seeds.
Seeds are fairly uniform things anyway, and the size of a seed as variable is as nothing to overall growth when compared with the variable of quality of the growth medium. A small acorn in rich soil will grow a bigger tree in a given amount of time than a big acorn in clay.
Cuttings are different. They aren't very uniform in starting size and also they are from established plants. They have stem structures in place like cambium and xylem. Some may even have lignified or lignifying cores. That difference can have a far more tangible effect on the outcome than the size of a seed.
If Fizz and Doodle grow at the same rate then Fizz will still be bigger than Doodle. If they grow proportionate to their starting size then Fizz will end up significantly bigger than Doodle. What is needed therefore is a starting measurement which is capable of being compared with the final measurement. To record the dry mass is to kill the specimen entirely. I cannot do that at the start for then there would be no experiment. The ashes of dead plants don't conveniently grow into new plants so that you can cremate them again in three months' time.
Dry mass is more accurate, more scientific, but it cannot be used in this instance. Length therefore is a useful proxy for how much mass the plant is accruing.