The grapevine tendril is located on the nodes of the shoot opposite to the leaves in a strict intermittent sequence: two adjacent nodes with tendrils are always followed by an empty node without a tendril. Morphologically, the tendril consists of a main lower axis and two primary ramifications. The shorter ramification terminates the main axis of the tendril itself, while the longer one is a second-order axis emerging from the axil of a leaf reduced to a scale (Fig. 17). Quite frequently, tendrils in various varieties possess three or four branches, yet they always maintain this sequential branching pattern.
Tendril Origin Theory and Transitional Forms
As early as the 19th century, scientists Palm and Mohl suggested that the tendril in the grapevine is a metamorphosed (modified) floral axis. In modern ampelography, the close connection between the tendril and the inflorescence has been conclusively proven. Practitioners are well aware of all the transitional stages between these two organs: quite frequently, tendrils bear anywhere from single flowers to several dozen full-fledged flowers, which subsequently develop into normal, mature berries. Even more often, groups of weakly differentiated, underdeveloped buds can be visually observed at the tips of young tendril ramifications.
According to the universally recognized Braun-Eichler theory of the sympodial structure of the grapevine shoot, each tendril is originally a reduced shoot apex. This apex is pushed aside by a vigorously developing axillary shoot, causing it to stop its linear growth and transform into a grasping organ (Fig. 14). Proof of this theory is provided by rare cases in certain grapevine varieties (e.g., Alburla) where so-called "extra-axillary" shoots with numerous true leaves and even inflorescences develop in mass instead of ordinary tendrils.
Tendril Movements and the "Grasping" Function
A young, herbaceous grapevine tendril initially grows straight, and only the very tips of its ramifications are slightly curved in an arc-like manner. This occurs because one longitudinal half of the tendril grows in length slightly faster than the other. Since the zone of intensive growth continuously moves in a circle around the tendril's axis, its branches describe distinct circumnutating "grasping" movements in space.
Interestingly, the rotation can occur both clockwise and in the opposite direction. According to precise observations by Charles Darwin, on a hot, sunny day, a grapevine tendril describes a complete, regular circle in an average of 2 hours and 15 minutes. As soon as the oscillating tendril touches any neighboring object or a trellis wire, its tip, due to specific mechanical irritation, begins to rapidly wrap around the support. Following this, the free part of the tendril twists spirally in the form of a tight spring, and its delicate herbaceous tissues quickly lignify. Thanks to this mechanism, the growing grapevine shoot attaches itself firmly to the trellis.
