For xylem I just drew a circle kinda thing with lignin thickening and for phloem I drew a sieve tube, perforated sieve plate, and a companion cell. Not sure what they're expecting since you can find so many different images for the transverse section of xylem and phloem online. Also about the hexagonal thing, I doubt that would be something marks will be taken off/awarded for, there are microscope images of xylem with a kind of round structure. I'm guessing the main thing is to note that xylem is thickened by lignin while phloem aren't and that phloem are living, xylem are dead at maturity.
For the question about marine fish and freshwater fish I had a similar question in my trial where I got full marks for writing (summarised)
- freshwater fish excrete dilute urine (have large glomeruli to filter large fluid volume)
- freshwater fish excrete nitrogenous waste in the form of ammonia (majority lost via gills due to close proximity with water, some ammonia is excreted in urine, salt concentration of urine is low due to active reabsorption)
- marine fish excrete conc urine to conserve water (nitrog waste excreted as urea, higher conc of salts in urine due to active secretion)
But in the HSC I basically just wrote the dilute urine vs concentrated urine thing and explained using some structures (ie large vs small glomeruli corresponds to volume of fluid filtered, and loop of Henle is long in marine fish counter-current multiplier system to increase reabsorption of water which makes urine very concentrated) but didn't mention ammonia vs urea. Could I still get 4 marks?