Though almost in all cases, when we talk about the origin, it goes about the discussion of fossils, bryophytes don’t have any primitive form than what at present-day it is. Usually, the study about the origin of bryophytes is something like a hypothesis- based upon the comparative studies about the morphology, habitat, and phylogeny of other living organisms. It is a belief in common that Bryophytes have originated from algae (except a few sectarian).
Chlorophyceae is the universal one that is proposed and expected widely as the origin of bryophytes. The basis before this is, Chlorophyceae is the only algae with identical chlorophyllous and xanthophyllus relatedly present in bryophytes.
There is another hypothesis like-
An oogamous alga migrated from water to land followed by the way of evolution of a sexual plant similar to bryophytes. Though it’s referred to as the alternation of generation- an important characteristic that bryophyte shows, not too much acceptable actually.
The fact is, algae need not be evolved into sexual plants while migrating from water to land ( and at the same time, there is no strong evidence in its court also). It may also be happened (evolution from isogamy to oogamy) after migration.
So the wiser way is to talk about the origin of gametophytes and sporophytes separately…
Origin of Bryophytes: Gametophyte
Different views have been proposed concerning the vegetative structure of the archaic bryophytic gametophyte.
The two main & diametrically opposed views are:
1. The gametophyte ( in the primitive stage) was nothing but an erect leafy plant.
2. The gametophyte was a simple thallus-like structure. No differentiation was developed at that time both internally & externally.
According to the first view,
the bryophytic gametophyte had performed a long jump to green algae.
Again, the second one is claiming that
parenchymatous terrestrial green algae are presenting the initial point of such evolution.
As both of the views are somehow indicating a similar case, whether it is upward or downward, we are now going to a comparative study.
– There seems no apparent relation between the antheridium and archegonium of bryophytes and the antheridium and oogonium of algae. The superficial cells of bryophytes are developed into a sterile jacket layer that is completely perceiving the fact of algae.
According to another hypothesis,
the antheridia and archegonia originated from the gametangia of Ectocarpos or something similar kind of this. But you will be shocked to know that, any green algae as long as studied till today, doesn’t contain such type of gametangia mentioned above.
Origin Of Bryophytes: Sporophyte
The two main theories about the origin of sporophytes are the following:
1. The Homologus theory
2. The Antithetic theory
1. Homologus Theory
According to the homologous theory, the sporophyte directly originated from the gametophyte.
Evidences supporting this theory are:-
– Isomorphic alternation in algae (it refers that, migration from aquatic to terrestrial environment is not at all essential to hold the characteristics of alternation of generation).
– Photosynthesis in sporophytes of bryophytes.
– Presence of tracheids in gametophytes of pteridophytes.
2. Antithetic theory
The sporophyte is a new structure inserted between two successive generations. Again, the elaboration of the zygote is the main fact in developing sporophytes.
It also shows the co-relation with those hypotheses- algae being the ancestor of bryophytes.
Since most of the discussions are hypothetical, we will be so glad to update these pieces of information with enough evidence whenever is available.