Cucurbita stem

 
The pumpkin family is one of a minority of plant families in which the phloem tissue occurs outside of (exarch) as well as on the inside of (endarch) the xylem. This micrograph illustrates part of a large vascular bundle of the stem of a Cucurbit. Notice the very large-diameter metaxylem vessels (MX) which is surrounded by small tracheids. Part of the cambium (CZ) can be seem above the metaxylem vessel, and this has produced several rows of secondary phloem, in which sieve tubes (ST) with characteristic sieve plates can be seen quite clearly in the detail below.