Many microorganisms survive such environmental stresses as heat, cold
and desiccation not by rapid rates of mitosis, but by producing specialized cells
designed to persist in a dormant state in hostile environments. Most fungi, for
Line example, yield single-celled spores, which, through wind distribution, can
(5) survive for long periods of time before germinating and sprouting fungal
filaments of their own. Other types of bacteria produce a special type of spore
called an endospore, capable of withstanding such extremes as boiling and
freezing temperatures, and even ultraviolet radiation.
Though research results remain tentative, several factors may protect
(10) endospores from environmental stress: they have a low water content, unusual
proteins and a tough spore coat absent in mature bacterial cells. When garden
fruits and vegetables, which may contain botulism endospores, are preserved by
canning at boiling temperatures, we know that these spores survive the heat
and sprout in the food, and as a result, the bacteria generate the botulism toxin
(15) that can lead to food poisoning—proof of their magnificent resilience.

The author's primary purpose in the passage is to（）