Question

name two type of roods
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Answer 1

Explanation:

• Taproots.
• Taproots.Fibrous roots.
• Taproots.Fibrous roots.Adventitious roots.

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Answer 2

Answer:

There are two types of photoreceptors in the human retina, rods and cones.

Rods are responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity.

Cones are active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity. The central fovea is populated exclusively by cones. There are 3 types of cones which we will refer to as the short-wavelength sensitive cones, the middle-wavelength sensitive cones and the long-wavelength sensitive cones or S-cone, M-cones, and L-cones for short.

The light levels where both are operational are called mesopic.

The bottom figure shows the distribution of rods and cones in the retina. This data was prepared from histological sections made on human eyes.

In the top figure, you can relate visual angle to the position on the retina in the eye.

Notice that the fovea is rod-free and has a very high density of cones. The density of cones falls of rapidly to a constant level at about 10-15 degrees from the fovea. Notice the blind spot which has no receptors.

At about 15°-20° from the fovea, the density of the rods reaches a maximum. (Remember where Hecht, Schlaer, and Pirenne presented their stimuli.) A longitudinal section would appear similar however there would be no blind spot. Remember this if you want to present peripheral stimuli and you want to avoid the blind spot.

Here is a figure from the textbook that shows the changes in the size of the photoreceptors with eccentricity. The bottom graph shows individual variations in the density of cones.

Here are schematic diagrams of the structure of the rods and cones:

This figure shows the variety in the shapes and sizes of receptors across and within species.

Here is a summary of the properties and the differences in properties between the rods and cones:

Properties of Rod and Cone Systems

Rods  Cones  Comment

More photopigment  Less photopigment  

Slow response: long integration time  Fast response: short integration time  Temporal integration

High amplification  Less amplification  Single quantum detection in rods (Hecht, Schlaer & Pirenne)

Saturating Response (by 6% bleached)  Non-saturating response (except S-cones)  The rods' response saturates when only a small amount of the pigment is bleached (the absorption of a photon by a pigment molecule is known as bleaching the pigment).

Not directionally selective  Directionally selective  Stiles-Crawford effect (see later this chapter)

Highly convergent retinal pathways  Less convergent retinal pathways  Spatial integration

High sensitivity  Lower absolute sensitivity  

Low acuity  High acuity  Results from degree of spatial integration

Achromatic: one type of pigment  Chromatic: three types of pigment  Color vision results from comparisons between cone responses

Explanation: