Figure 7.31d on page 148 shows a sieve element with red-stained ‘triangles’ of callose at each end. These triangles indicate the positions of the sieve plates.

a. Assuming the magnification of the micrograph is × 100, calculate the length of the sieve element. Show your working. (You may find it useful to refer to Chapter 1, Worked example 4, page 9, for the method of calculation.)

b. Scientists were puzzled for many years by the fact that sieve plates were present in sieve elements, because sieve plates increase the resistance to flow. This contrasts with xylem vessel elements, which have open ends, reducing resistance to flow.

i. Calculate how many sieve plates per metre a sucrose molecule would have to cross if it were travelling in the sieve tube identified in a above. Show your working. (Assume all the sieve elements are the same size as the one measured in Figure 7.31d.)

ii. What is the function of the sieve plates?

iii. What feature of the sieve plates allows materials to cross them?

c. Flow rates in sieve tubes range from 0.3 to 1.5 m h⁻¹ and average about 1 m h⁻¹. If the flow rate in the sieve element shown in Figure 7.31d were 1 m h⁻¹, how long would it take a sucrose molecule to travel through it? Show your working.

Figure 7.31

Transcribed Image Text:

a b parenchyma cell nucleus of parenchyma cell sieve plate with sieve pores companion cell sieve tube cell walls of neighbouring cells sieve plate - sieve pore middle lamella nucleus -companion cell dense cytoplsm sieve tube element - relatively large lumen with little structure visible thin peripheral layer of cytoplasm (no nucleus)