Question

why salvinia does not have prothallus

Answer 1

Prothallus In Salvinia, both male and female gametophytes are found in massulaes not in prothallus.
Salvinia- prothallus is wrongly matched pair. ... is because both male and female gametophytes in Salvinia is found in structures called massulaes and not prothallus

Answer 2

Salvinia does not have prothallus because it is heterosporous in nature.  

Explanation:

In most of the Pteridophytes the spores formed by meiosis in spore mother cells gives rise to a gametophyte called Prothallus. It contains “both male and female sex organs called antheridia and archegonia”. With help of these organs reproduction occurs.

On the other hand species like Salvinia, being heterosporous in nature they don’t require gametophyte as they themselves releases two different type of spores namely megaspores and microspores. The megaspore germinates as female and the microspores germinate as male. Hence Salvinia does not have prothallus.