Stage thymocytes express neither CD4 nor CD8, markers associated with mature T cells. They are therefore called double negative (DN) cells. DN cells first travel to the region under the thymic capsule, a region referred to as the ________, where they proliferate and begin to generate their T-cell receptors. Thymocytes that successfully express TCRs begin to express both CD4 and CD8, becoming double positive (DP) cells, and populate the cortex, the site where most (85% or more) immature T cells are found.

#Question id: 9174

#Unit 9. Diversity of Life Forms

Use the following figures to answer the question below.

Based on the data in the figures, which of the following statements is accurate?

#Question id: 9173

#Unit 9. Diversity of Life Forms

Use the following figure to answer the question below

In the figure, which number would designate the arctic tundra biome?

#Question id: 9171

#Unit 9. Diversity of Life Forms

Suppose that the number of bird species is determined mainly by the number of vertical strata found in the environment. If so, in which of the following biomes would you find the greatest number of bird species?

#Question id: 9169

#Unit 9. Diversity of Life Forms

In areas of permafrost, stands of black spruce are frequently observed in the landscape, while other tree species are noticeably absent. Often these stands are referred to as "drunken forests" because many of the black spruce often "lean over" (that is, they are displaced from their normal vertical alignment). What is the most likely explanation for the unusual growth of these forests in this marginal habitat?

#Question id: 9164

#Unit 9. Diversity of Life Forms

Use the following figures to answer the question below.

East of the Mississippi River, in Hopkinsville Kentucky, the mean annual precipitation is 130 cm. The mean annual temperature is 14.3°C. In the winter, the mean temperature is 7.3°C, while in summer it is 20.6°C. Using the climograph shown here, how would we classify the biome found in Hopkinsville?

#Question id: 9163

#Unit 9. Diversity of Life Forms

Use the following figures to answer the question below

Based on the data in the figures, which of the following statements are correct?

I) Area 1 would be considered a desert because of its high average temperature.

II) Area 1 has more average precipitation than Area 2.

III) Area 2 would be considered a desert because of its low average precipitation.

IV) Area 2 has a larger annual temperature variation.