Reference:

1.“Protonema.” Encyclopædia Britannica, Encyclopædia Britannica, inc. Available here 
2.The Editors of Encyclopædia Britannica. “Prothallium.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 3 Dec. 2014. Available here 

Image Courtesy:

1.’Physcomitrella Protonema’By Anja Martin, Labor Ralf Reski  (CC BY-SA 1.0) via Commons Wikimedia 
2.’Prothallus (Gametophyte) (8231437739)’By Paul K from Sydney, Australia – Prothallus (Gametophyte), (CC BY 2.0) via Commons Wikimedia