The structure and development of P‐protein have been studied in sieve elements of hypocotyl tissue of Ecballium elaterium and Cicer arietinum, and in P‐protein‐producing cells of root apices of Polygonum fagopyrum. Ultrastructural investigations have led us to propose a model for the structure of P‐protein tubules. A tubule appears as a Super‐Double Helix (“DH1”) which consists of two 6‐ to 9‐nm‐diam strands wound round a central lumen, each strand exhibiting a varying‐pitched minor double helix (“DH2”). Our observations provide additional insights into the developmental relationships between the different forms of P‐protein and support the idea that spiny vesicles participate in P‐protein formation. The different types of P‐protein bodies found in mature sieve elements of species we have investigated may be regarded as arrays of axially oriented linked “DH1”