Cuscuta reflexa, intrusive phase. Host, Pelargonium zonale. Note collapsed zones in the adhesive disk (upper left and right).
Heide-Jørgensen H S. 1991. Notes on the structure of the adhesive disk of Cuscuta. In: Ransom J K, L J Musselman, A D Worsham, and C. Parker (eds). Proc. 5th Inter. Symp. Parasitic Weeds. Nairobi: CIMMYT. p 513.


Abstract/Summary:
Developing haustoria of Cuscuta reflexa Roxb. and C. gronovi Willd. (Cuscutaceae) initiated on pith parenchyma of Sambucus nigra L. (Caprifoliaceae) and on live stems of Pelargonium zonale (L) Alt. (Geraniaceae) were analyzed by light microscopy. The epithelial trichomes of the adhesive disk (Heide-Jørgensen, 1987 - Abstract 10) only become folded at their tips if the elongation growth of the trichomes is impeded by obstacles like the surface of a host. The meristem of the intrusive organ originates in the third cortical layer inside the epidermis. Under natural conditions, a 'prehaustorium' is not formed. A short lived glandular tissue develops in front of the developing meristem. Prior to penetration of the intrusive organ, the gland secretes large quantities of carbohydrate material, which are stored between the plasmalemma and the cell wall and to a lesser degree in the central vacuole. As penetration proceeds, the gland is obliterated, and the remains, together with compressed epithelial and hypodermal cells, line the interface between host and the intrusive organ to a depth of several cell layers of the cortex of the host. Concomitantly with the penetration of the intrusive organ, collapsed zones are formed.

REFERENCE
Heide-Jørgensen, H. S. 1987. Changes in cuticle structure during development and attachment of the upper haustorium of Cuscuta L., Cassytha L., and Viscum L. In Weber, H. C. and W. Forstreuter, editors. Parasitic Flowering Plants. Proceedings of the Fourth International Symposium on Parasitic Flowering Plants. Marburg: Philipps-University. pp 319-334.

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