Grandpa by James H. Schmitz (Astounding, February 1955) opens with a fifteen-year-old called Cord anesthetising and examining bugs on an alien planet called Sutang. He is then interrupted by Grayan, an older female friend who warns him that, if he doesn’t start behaving in accordance with the colony’s rules and expectations, he is likely to be sent off-planet.
After this YA setup to the story, the next part sees Cord, Grayan, Nirmond (the regent of the planet), and a young woman called Dane (head of the visiting Colonial Team) set off on a tour of the Bay Farms. To travel there they use one of the planetary life-forms:
Three rafts lay moored just offshore in the marshy cove, at the edge of which Nirmond had stopped the treadcar. They looked somewhat like exceptionally broad-brimmed, well-worn sugarloaf hats floating out there, green and leathery. Or like lily pads twenty-five feet across, with the upper section of a big, gray green pineapple growing from the center of each. Plant animals of some sort. Sutang was too new to have had its phyla sorted out into anything remotely like an orderly classification. The rafts were a local oddity which had been investigated and could be regarded as harmless and moderately useful. Their usefulness lay in the fact that they were employed as a rather slow means of transportation about the shallow, swampy waters of the Yoger Bay. That was as far as the team’s interest in them went at present.
They then go looking for “Grandpa”, a bigger raft they’d rather use but, when they eventually find it, Cord sees that it has moved from where he last left it. They also find that Grandpa’s head (a cone shaped protuberance in the middle of the raft) now has red buds on the top, and has also sprouted vines. Cord attempts to warn the others about using the raft as they have never seen this phenomenon before, but he is fobbed off.
The rest of the story sees an uneventful passage until they pass a group of yellowheads (“vaguely froggy things, man-sized and better”) clinging to tall reeds when, uncharacteristically, one of them slips down into the water and swims underneath Grandpa. Shortly after this event they lose control of the raft (it won’t respond to the heat from their guns) and then there is a convulsion that sees all of them except Cord trapped by the vines. Cord is subsequently forced, after a brief conversation with Dane, to relieve their pain by using his gun’s anaesthetic darts.
The rest of the story sees Grandpa travel far out to sea while Cord observes the creature’s behaviour. Eventually (spoiler) Cord manages to distract Grandpa (it has been swiping at various forms of life that pass before feeding on the smaller ones), and he jumps into the sea ahead. Cord then swims underneath the creature and manages to access a hollow space inside the central cone: there he finds the yellowhead symbiotically attached to Grandpa.
After fighting and killing the yellowhead, Cord slips back into the water and emerges to the rear of a now stationary raft. When he gets on board again it responds to his heat gun and the raft heads towards the shore.
For most of its length this reads like a rather dull YA biology puzzle, but it improves with an exciting climax. I’d note, however, that there is little indication of what Cord is about to do before he goes into the water (I can’t remember any description of him thinking about the yellowhead, or what he plans to do). This is rather too straightforward a piece, I think (especially for its length).
** (Average). 9,050 words. Story link.