Anatomy of the phloem and xylem in the vascular sutures of the soybean pod

Low temperature scanning and transmission electron microscopy were used to observe the anatomy of vascular tissue of the placental and nonplacental sutures of the carpel (pod) of soybean [Glycine max (L.) Merr.]. Pods were harvested when they reached their greatest length and then weekly until physiological maturity. The vascular tissue lies internal to a layer of thick-walled sclerenchyma tissue that is not connected to the sclerenchyma tissue of the pod pericarp. The walls of the sclerenchyma tissue become thicker with maturation. The vascular tissue is arranged with phloem external to the xylem. Phloem and xylem cells differentiate continuously throughout the period of seed development. The mature xylem consists of tracheary elements with helical secondary wall thickenings. The mature phloem consists of sieve elements and adjoining companion cells. The sieve elements did not form P-protein crystals. The companion cells were replete with mitochondria and the number of companion cells per sieve element varied from one to three or four. An unusual feature of the placental vasculature was the presence of an inner layer of phloem cells consisting of sieve elements and companion cells. These cells were similar to and joined with the outer phloem when they entered the funiculus (seed stalk). The presence of the inner phloem suggests that it may have a special function in the nourishment of the developing seed.