Question

Selaginella and Salvinia are considered to represent a significant step forward in the evolution of seed habit because
(a) Megaspore possesses an endosperm and embryo surrounded by a seed coat.
(b) The embryo develops in the female gametophyte and is retained on parent sporophyte.
(c) The female gametophyte is free and gets dispersed like a seed.
(d) Female gametophyte lacks archegonia.

Answer 1

Hint: Heterospory is considered to be a significant step in evolution in the case of Selaginella and Salvinia, where megaspore germinates on the parent sporophyte even after fertilization.

Complete answer:
Most of the pteridophytes produce a single type of spores thus called homosporous. However, in some pteridophytes like Selaginella and Salvinia two different types of spores were seen which were performing different functions such as pteridophytes were called heterosporous and the phenomenon is called heterospory.
In heterospory, two types of spores namely microspore, and megaspore are formed.
Microspores are smaller in size and develop into male gametophyte while the megaspore is larger and develops into the female gametophyte.
The phenomena of heterospory are of great biological significance because-
The female gametophytes in these plants were retained on the parent sporophyte for a variable number of times. The formation of the zygote into a young embryo takes place within the female gametophyte.
Same is true for the microspores they also start germinating into male gametophytes while they are still inside microsporangium
So, the correct answer is, ’ The embryo develops in the female gametophyte and is retained on parent sporophyte.’

Note:
The origin of seed habit is associated with:
- Formation of two types of spores (heterospory) .
- Decrease in the number of megaspores finally to one per megasporangium.
- The female gametophytes are retained on the parent sporophyte and the formation of the zygote into a
  a young embryo takes place within the female gametophyte.