Question

suspended pollen grains of pinus leads to respiratory allergies for tourists to Himalayas?​

Answer 1

Explanation:

The microspore or pollen grain at the time of shedding may have only one prothallial cell (Cycas, Ginkgo and Ephedra among Gnetales) or two prothallial cells (Coniferales), the former being liberated at three-celled stage and latter at four-celled stage. Also, there is no antheridium in which the sperm are produced. A grain of pine pollen also has two large air sacs to make it buoyant in the wind

So, the correct answer is 'four celled stage'