litres of water in its pith cavity with an enormous heat capacity: At an exterior temperature of -6 °C only about 2% of this liquid was frozen and only 1.8% of the heat storage capacity was made use of. The cistern-like bases of the rosette leaves also collect litres of water, which is only partially frozen after cold nights and thus constitutes a considerable heat reservoir (Fig. 1.3.31C). Investigations have shown that all of the leaves of a giant rosette (except, seemingly, the leaves of the genus Espeletia), including those still in the bud, can tolerate freezing. What is, then, the value of the avoidance of freezing? Daily de- and rehydration processes evidently put a great strain on tissues: Outer rosette leaves that more or less freeze every night age relatively quickly. Tips of leaves which are exposed to the sunlight in their frozen state yellow after only a few days. It therefore appears to be ad vantageous for tissues to avoid freezing stress even when they are frost-hardy.
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