Our studies reveal that Arceuthobium globosum, a primitive member of the genus, has highly specialized phloem tissue. The sieve elements of this large Mexican species have transverse to oblique, usually simple sieve plates and small but numerous diffuse lateral pores. The sieve elements show the normal pattern of callose deposition and subsequent removal reported for other dicotyledons. Sieve elements were not found in A. douglasii or A. tsugense, two specialized, temperate-region species.