How can you tell if an exponential model describes growth or decay? Give examples. Solution An exponential model is A = Pekt where P is the initial quantity/amount, A is the quantity after time t and r is the constant of growth/decay. If k is positive, the model depicts growth and if k is negative, the model represents decay. In a case of continuous compounding of interest, k will be positive and will denote rate of interest. Here k > 0 and the example is of exponential growth. Similarly, in a case of bacterial growth, k will be positive and the model will represent exponential growth. However, in the case of a radioactive substance, where the matter decays over time, k wiill be negative and the model is of decay..

1. How do the megasporangium and megaspore in seed plants differ from primitive heterosporous
plants? Explain.
Solution
Pteridophytes are mostly homosporous except few (Selaginella,Salvinia, Marselia). Those are
considered as primitive heterosporous plants that shows some of the seed habits. The seed plants
show distinct heterospores (micro and megaspores). Megasporangium produces megaspores,
which further develop into female gametophyte. Microsporangium produces microspores, which
further develop into male gametophyte. There are few differences between the megaspores
between primitive heterosporous plants and seed plant.
In primitive plants:
1.The female gametophyte bear archegonium
2.Only 4 megaspores are formed totally. That is only one mother cell becomes functional.
3.Megagametophyte is shed only after embryo formation.
4.Nutritive tissue called tapetum is formed around megasporangium
5. The megaspore develop a cell wall; outer thick exospore and inner thin endospore
6. No micropyle
In seed plants:
1.Mega gametophyte is observed
2.Each megaspore mother cell in the megasporangium on meiosis will give rise to 4 megaspores.
Greater chance for healthy gametophytes
3.Megagametophyte is retained
4.Protective layers like Integuments and nucellus are formed around megasporangium
5.Embryo sac is formed around megaspore.
6. Micropyle is seen. It's the point of contact for pollen tube.
7.