Re: How are roots different when they are produced in water?

I can't answer all your questions but can provide some general information and 
some references where you might find more info. Plants rooted in water produce 
so-called "water roots" which do differ from soil-grown roots. They sometimes 
seem to be more brittle than normal roots. It may be that the brittleness is 
due to greater aerenchyma, or tissue with a greater percentage of intercellular 
air spaces than normal parenchyma.If you want to eventually plant your rooted 
cuttings in soil, it is usually not recommended to root them in water because 
of the frequent difficulty in transplanting them to soil.

I don't know if there is much literature on water roots. If there is, it is 
probably from the early 1900s or older. I'm not sure there is a more specific 
term than "water root." It would make searching easier if there were a more 
specific term. The standard college Plant Propagation textbook by Hartmann and 
Kester has a great deal of info and references on root initiation in cuttings. 
Also try the books by Hewitt (1966) and Kramer and Boyer(1995) who may discuss 
water roots. The webpage on aerenchyma also has some citations that might be 
useful.

In some plants, water roots seem to survive when the plant is planted in soil. 
Other plants with water roots transplant poorly. New roots usually originate in 
the pericyle but there are exceptions. Many plants rooted in water, such as the 
houseplants wandering jew, pothos and philodendron have preformed root initials 
on their stems that merely elongate when cuttings are placed in water. 

Early researchers in water culture in the mid-1600s found that cuttings could 
survive for up to 6 or 7 years in water culture. Spearmint was one of the 
favorites of the early water culture researchers such as Robert Boyle, Thomas 
Browne, Robert Sharrock and John Woodward. Plant longevity in water culture 
will depend on the water source and the size and type of cutting used. The more 
essential mineral nutrients the water contains the better. Water culture was 
replaced with solution culture or hydroponics in the 1800s after the mineral 
nutrient requirements of plants were recognized. 

What might be happening with root death and decay in water culture is that 
roots become deficient in mineral nutrients, particularly calcium and boron, 
which are required in the external solution for normal functioning of roots. As 
some roots die and decay, they release some mineral nutrients, which then 
presumably allows new roots to grow. I grew pothos and philodendron in a 
mineral nutrient solution that was lacking calcium and noticed that pattern 
repeat several times. Other stresses on roots in plain water culture is that 
they may suffer from lack of oxygen and pH extremes.

References


Hershey, D.R. and Ku, C.S.M. 1991. Calcium deficiency symptoms of Epipremnum 
aureum.  HortScience 26: 606. 

Hershey, D.R. 1994. Solution culture hydroponics: History and inexpensive 
equipment. American Biology Teacher 56:111-118. 

Hewitt, E.J. 1966. Sand and Water Culture Methods Used in the Study of Plant 
Nutrition, rev. 2nd edition. Commonwealth Bureau of Horticulture and Plantation 
Crops, East Malling. Tech. Communication Number 22. 

Kramer, Paul J. and  Boyer, John S.1995. Water Relations of Plants and Soils. 
NY: Academic.

Webster, C. 1966. Water as the ultimate principle of nature: The background to 
Boyle's Sceptical Chymist. Ambix 13: 96-107.

Woodward, J. 1699. Some thoughts and experiments concerning vegetation. 
Philosophical Transactions of the Royal Society 21: 193-227.


Aerenchyma