"Look at this," he said, thrusting a dusty book under Mendel's nose. "the Tatian heresy as found in Apolinarius, and his address to the Emperor. I have never trusted his writings against the Phrygian heresy, I've always considered it very contrived. What are you doing?"

"I'm not sure," his friend told him, brushing away some of the dust that had fallen from Brother Joseph's ancient text. Mendel could understand intellectual research, but he had never understood his friend's obsession with mystical points of debate about arcane issues long dead and forgotten. "But I hope to know more in a few minutes when I've examined this mud under the microscope."

"What mud?"

So Mendel explained where he had found the piece of dried mud and why he considered it important. "As near as I can tell, this mud comes from the boot of the arsonist and saboteur. The fire hearth was cleaned that morning, and none of the people who went into the room later stepped into the hearth or onto the tiles. But the person who broke the Krautstrunk beaker certainly did, and left this as a clue."

It was Brother Joseph's turn to be mystified. "But what can a cake of dried mud tell you about anything?" he wanted to know. "Even if the saboteur left it there, it will hardly have his name on it."

"Perhaps not," Mendel had to agree, "but you would be surprised what you find in a sample like this. When I was at the University of Vienna, one of our research projects was to go out to local ponds, collect samples, look at them under the microscope, and then catalogue what we found. When we showed our results to Herr Professor Doktor Unger he could instantly tell us from which pond, and in some cases from which end of the pond we had collected the samples.

"At that time he was under orders from the ministry to undertake a microscopic study of the internal structure of plants, but he always stressed how we should use inductive methods to come to reasonable conclusions, based on the evidence, and not use the old fashioned empirical methods." He paused, took out the mud sample, and meticulously moved a small part of it onto a glass dish.

"I was also influenced by Professor Schleiden's book on the Principles of scientific botany, which we used in classes. I remember one line very vividly, '--- a complete theoretical explanation, in which we explain the connection of facts that are subject to laws in terms of the latter, is possible only on the basis of mathematics and only in so far as mathematical treatment is feasible'. He insisted that we work by collecting the facts, then use rational inductive reasoning to arrive at a conclusion."

"But how does that help you when looking at mud?" Lindenthal wanted to know.

"You'd be surprised what you find in a mud sample like this one," Brother Gregory said, adding a little water to the mud on the glass and stirring it slightly. "Small grains of sand and flint can tell you what roads the boot walked on, the presence of grass fragments can tell you which fields the boot walked on, and if there is a lot of leaf debris, the boot was worn in a wood or in bushes. We may be able to build up a complete history of this person's last day's travel just by looking at what is here."

Together the friends began to examine the suspended material from the mud. At first it was not very enlightening. Most of what they saw was little more than opaque grains of various minerals, which, despite what Mendel had said, were generic and found all over Brno.

"Let's try and find some organic material," Mendel said at last. He shook the slurry of mud in water, allowed it to settle out and then decanted a small part of the cloudy liquid onto a series of microscope slides. Here they were more successful and, as he had said, Brother Gregory found traces of grass fragments, leaf debris, some tiny microorganisms and then, just when he was about to give up, a clump of very small, round highly refractive spheres.

"Ah!," he exclaimed to Lindenthal, who had become bored and gone off to read his book by Mendel's window. "Now this is something interesting, and possibly of use."

Brother Joseph returned to the microscope and was shown the tiny spheres. "What are they?" he asked, rubbing his eyes. "Can't you make them any bigger?"

So Mendel changed the objective lens on the microscope to one that Rosenstrauch had just tested, and looked for the spheres again. "I don't think they are microorganisms," he said at last, "I've seen things like this before. I'm sure they are the male pollen grains from a plant."

"Pollen?" said Lindenthal.

"Yes, the male reproductive structures of all the plants that have been tested, produce tiny particles of dust, very like these, from special structures in the flower. They carry the male principle of heredity - or at least they do in my pea plants, of that I am certain. This 'dust' from the male joins somehow with larger structures in the female parts of the flower to produce the zygote and seed."

He walked over to his bookshelf and took down one of the large tombs, propped it on the back of a chair, and spent some minutes finding the right section. "Look at this," he said eventually, and Brother Joseph saw an illustration plate of drawings, all of which were carefully labeled spheres similar to the ones they had just observed down the microscope.

"Pollen," said Mendel emphatically, "I've seen it before. When I do my hybridization studies I use a small paintbrush to move pollen from the male flower structures to the receptacles on the female flower structures. I once looked at this pollen under the microscope, just as we are doing now. I made a drawing - look."

He showed his friend his sketch of Pisum pollen.

"But none of the drawings look anything like these things from the mud," Brother Joseph insisted. Indeed they did not, and Mendel could not deny it. "In fact," he continued, "none of these pollen grains look alike - they are all different."

A smile spread across Mendel's face and he pulled his spectacles down to the end of his nose. Looking over their rims at his friend, he said, "Perhaps that is exactly what we want! If all plants produce different shapes of pollen grains, then we should be able to identify the plant that produced the pollen trapped in this mud. When we know that, we will know where the boot has been recently. It could be a valuable clue!"

"I'm not sure why," Lindenthal hesitated. Brother Gregory sometimes became too enthusiastic. "Even if you are right and we can identify that pollen, how does that help us? I would bet that we will find exactly the same pollen on the feet of everyone in and out of the monastery."

Some of Mendel's excitement drained away and he turned back to the microscope. Pulling a piece of paper up beside him he began to draw pictures of the pollen grains he saw. "Perhaps," he said slowly, "but at least it give us something to do, and I must confess, I am at a loss to know what to do next."

Brother Joseph hesitated, the two friends had not discussed the central issue. "Do you think he did it?" he asked finally, and had to wait a long time for an answer.

"I don't want to," Brother Gregory said at last, "I've known Brother Matthew for a long time and I would like to think he is not capable of something like this. But I have to admit, the evidence is very damming, and it is hard to see how anyone but Klacel could have started that fire on the desk."

They fell silent for a moment while they both considered the consequences of a friend who was in trouble and their own feelings.

"So, let's get some of the mud from his boots and see if it matches," Lindenthal said at last.