Mention the most significant function of each of the following.
(i) Tapetum cells
(ii) Serotonin
(iii) Lenticels
(iv) Cerebrospinal fluid
(v) Islets of Langerhans
(vi) Bundle sheath

(ii) Serotonin is a neurotransmitter which is acetylated and then methylated within the pineal gland to produce melatonin. 

(iii) Lenticel is a porous tissue which provides a direct pathway between the internal tissues and atmosphere through the bark, but impermeable to gases.

(iv) Cerebrospinal fluid acts as the cushion for the brain and acts as the shock absorber for the central nervous system.

(v) Islets of Langerhans is also called pancreatic islets contain many cells including the beta cells, that secreted insulin.

(vi) Bundle sheath is the layer of compact cells surrounding the vascular bundle (in a plant) that regulates the movement of substances between the vascular tissue and the parenchyma and in leaves protect the vascular tissue from exposure to air.