У фикуса крепкий стебель, а у орхидеи длинный корень.

In Russian, у + Genitive is a very common way to express possession or a characteristic.

So:

• У фикуса крепкий стебель literally means something like At the ficus, there is a strong stem
• Natural English: The ficus has a strong stem

And:

• У орхидеи длинный корень = The orchid has a long root

Russian often prefers this structure instead of using a direct equivalent of to have in simple descriptions.

Because after у in this meaning, the noun goes into the genitive case.

Dictionary forms:

• фикус → фикуса
• орхидея → орхидеи

So:

• у фикуса
• у орхидеи

This is standard Russian grammar: у + genitive for possession.

Russian usually does not use a present-tense verb meaning has in this kind of sentence.

Instead of saying:

• The ficus has a strong stem

Russian says:

• У фикуса крепкий стебель

There is no separate present-tense verb here. The idea of has is understood from the structure у + genitive + noun.

You may sometimes see есть, but here it is normally omitted:

• У фикуса есть крепкий стебель is grammatical, but less natural in this descriptive context.

Because they are the things being talked about as existing or belonging to the plant.

In this pattern:

• у + possessor in genitive
• thing possessed in nominative

So:

• у фикуса = of the ficus / belonging to the ficus
• крепкий стебель = a strong stem

And:

• у орхидеи = of the orchid / belonging to the orchid
• длинный корень = a long root

That is why стебель and корень stay in the nominative singular.

Russian adjectives must agree with the nouns they describe in gender, number, and case.

Here:

• стебель is masculine singular nominative → крепкий
• корень is masculine singular nominative → длинный

So the adjective endings match the nouns.

If the nouns were feminine, neuter, plural, or in another case, the adjective forms would change too.

А often links two clauses with a contrast or comparison.

In this sentence, it is something like:

• while
• whereas
• and, by contrast

So:

• У фикуса крепкий стебель, а у орхидеи длинный корень = The ficus has a strong stem, whereas the orchid has a long root

It is not as strong as but in many contexts. Here it mainly sets the two plants side by side for comparison.

Here орхидеи is genitive singular.

The dictionary form is:

• орхидея

This is a feminine noun ending in -я. In the genitive singular, many such nouns change to -и:

• орхидея → орхидеи

So after у, you get:

• у орхидеи

This form can look a little confusing because орхидеи can also be another case/number form in other contexts, but here it is definitely genitive singular because it follows у.

Yes. Russian word order is flexible, although the original order is very natural.

Original:

• У фикуса крепкий стебель, а у орхидеи длинный корень.

You could also say:

• Крепкий стебель у фикуса, а длинный корень у орхидеи.

That version puts more emphasis on крепкий стебель and длинный корень.

So the word order can change, but the original is the most straightforward neutral way to present the information.

Yes, in this sentence you normally repeat it:

• У фикуса ... , а у орхидеи ...

Each clause has its own possessor, so repeating у makes the structure clear and natural.

If you said:

• У фикуса крепкий стебель, а орхидеи длинный корень

that would be incorrect.

Russian often uses the singular when talking about a typical body part or structural part of something.

So:

• крепкий стебель = a strong stem
• длинный корень = a long root

This sounds like a general description of each plant.

Plural forms would be possible only if the meaning really required them, for example if you wanted to emphasize multiple stems or roots:

• У растения длинные корни = The plant has long roots

But in your sentence, the singular is the natural descriptive choice.