AN INVESTIGATION OF THE FUNCTIONAL PHLOEM IN WILLOW

SUMMARY Distribution of dyes, 14C, and the position of aphid stylets in the phloem of rooted 2 year cuttings and a standing tree of Salix fragilis correlate with callose formation on the sieve plates. It is suggested that the region concerned with translocation is that portion of the phloem near the cambium, where callose is formed on the sieve plates. During July this region is within the first lignified fibre band where there are sieve tubes in all stages of development, from recently differentiated to those in which definitive callose covers the sieve plate. The majority of sieve tubes in the area have open sieve pores with callose between the pores, and with sparse fibrillar or vesicular cytoplasm.