Simulation · Biology · Class 11
Does a pine need a second tree?
From the lesson Gymnosperm cones, pollen and ovules in Plant Kingdom. Change the values and watch what happens.
Does a pine need a second tree?Biology · Class 11
The idea behind it
NCERT §3.4
- Gymnosperms are heterosporous and make haploid microspores and megaspores, inside sporangia borne on sporophylls. The sporophylls spiral along an axis to form loose or compact strobili (cones).
- Cones with microsporophylls and microsporangia are male (microsporangiate) strobili. Each microspore becomes a male gametophyte reduced to a few cells, the pollen grain, which develops inside the microsporangium.
- Cones with megasporophylls carrying ovules (megasporangia) are female (macrosporangiate) strobili. Pinus bears male and female cones on the same tree; in Cycas, male cones and megasporophylls are on separate trees.
- One cell of the nucellus becomes the megaspore mother cell. The nucellus wrapped in its protective envelopes is the ovule.
- The megaspore mother cell divides by meiosis into four megaspores. One of them, still inside the megasporangium, grows into a multicellular female gametophyte bearing two or more archegonia, and this gametophyte too stays inside the megasporangium.
- Unlike bryophytes and pteridophytes, neither gametophyte of a gymnosperm lives independently; both stay within sporangia on the sporophyte.
- Pollen grains released from the microsporangium are carried by air currents to the opening of the ovules. A pollen tube carrying male gametes grows towards the archegonia and releases them near the archegonium's mouth.
- After fertilisation the zygote becomes an embryo and the ovule becomes a seed, which stays uncovered.