I didn't understand anything in that sentence.

By treating cuttings with a colchicine mixture at just the right concentration, half of them will root, while the others die from poisoning. Those that make it have had their cytoplasm modified, giving rise to chromosome doubling.

These cuttings are set out, and the axillary growth observed for unusual shoots, which are cut off and rooted. These will become the tetraploid children. Or at least, that's how I remember it.

It may be possible to apply the colchicine to the incipient axillary shoots at the nodes, and there are certain herbicides that are more accessible than colchicine that will do the same at subleathal doses.