Table II. Frequency of PD per cellular interface in the veins of wild-type and tdy2 yellow leaf tissues.

The values represent means ± sd. No statistical differences were observed between wild-type and tdy2 yellow leaf tissues as determined using Student’s t test. In wild-type maize minor veins, two types of SEs are present: thin-walled SE (SE) and thick-walled SE (thSE; Evert et al., 1978). The SEs are connected to the CC and function in Suc phloem loading. The thSE are connected to VP cells and are postulated to be involved in the retrieval of Suc leaked to the apoplast (Fritz et al., 1983). The number of interfaces analyzed for wild-type and tdy2 yellow tissues were 822 and 893 for the BS-VP interface, 680 and 853 for the VP-VP interface, 279 and 276 for the thSE-VP interface, and 396 and 548 for the thin-walled SE-CC interface, respectively. Differences in the number of cellular interfaces are due to the preponderance of some cell types (e.g. BS cells) relative to the scarcity of others (e.g. thSE; Evert et al., 1978).

	No. of PD per Interface		No. of PD per Pit Field		No. of Pit Fields per Interface
Interface	Wild Type	tdy2	Wild Type	tdy2	Wild Type	tdy2
BS-VP	3.0 ± 0.2	2.8 ± 0.3	6.1 ± 0.1	6.2 ± 0.8	0.49 ± 0.05	0.45 ± 0.08
VP-VP	2.0 ± 0.3	1.8 ± 0.4	5.2 ± 0.2	4.8 ± 1.3	0.38 ± 0.04	0.38 ± 0.14
thSE-VP	0.40 ± 0.24	0.34 ± 0.13	1.8 ± 0.4	2.5 ± 1.2	0.21 ± 0.08	0.14 ± 0.04
SE-CC	0.25 ± 0.10	0.27 ± 0.06	2.4 ± 0.3	2.8 ± 0.9	0.10 ± 0.04	0.10 ± 0.02