Re: do trees take in water during the day or night

Trees and plants in general can "take up" water both during day and night. 
However, the process is affected by light or dark because stomata, the tiny 
openings in the leaf surface through which water evaporates, usually close in 
the dark and open in the light. Environmental factors also determine how much 
water is acquired. The primary factors are the available water in the soil and 
the amount of water in the atmosphere in relation to how much water the air can 
hold (relative humidity), which changes with temperature.  

Plants take up water and transport it to their leaves by a combination of 
three forces.  Root cells absorb water from the surrounding soil, passing 
the water from cell to cell and then into the xylem cells, which you can 
think of as dead tubes.  Living cells of the roots can generate a pressure 
pushing the water up the xylem tubes.  You can demonstrate this by taking a 
well-watered potted plant, a carrot, a tomato, a coleus, a geranium, and 
cutting off the leafy crown leaving a naked stem.  If you connect a clear 
tube to the cut end of the stem (keep the end wet).  You can observe the 
water pushed up from the roots rising in the tube and you can measure this 
by marking the level at time intervals.  This is root pressure.  

Water also moves up through xylem cells because of the attraction between 
water molecules and the sides of the xylem cells.  If you put a slender 
tube, like a straw or pipette, into water, you will notice that the water 
rises a little above the surface, and it will rise more in narrower tubes. 
Since xylem cells are very narrow, this force helps pull water upward and 
is called capillary action.  A capillary is a narrow tube.

However both root pressure and capillary action can only account for water 
rising a few feet above ground.  Clearly the leafy crowns of trees tower 
above this height. The remaining force accounts for the rest of the take up 
of water.  Plants constantly are losing water into the atmosphere, a 
process called transpiration.  You can show this by placing a plant inside 
a jar.  If the pot is sealed inside a plastic bag, the condensation that 
forms on the inside of the jar comes from transpiration.  If a plant loses 
water faster than it is replaced, the plant wilts, but transpiration is not 
a bad thing.  Water molecules have a strong adhesion, they tend to stick 
together, so when water molecules are pulled into the atmosphere they exert 
a tug, a pull, on the entire column of water in the xylem cells.  
Transpirational pull generates most of the force that moves water to the 
crowns of tall plants.

Your experiment with celery was OK.  The movement of dye up the celery 
stalk, which is only a leaf, would demonstrate plants take up more water in the 
light than in the dark.  If you used a container that trapped the transpired 
water, the humidity would go up for the plant, and the transpirational water 
loss would be less, and so would the movement of dye up the stalk.  

Outside at night, as the air cools the amount of water it can hold decreases. If 
the air cools enough, the water in air will condense to make dew.  At 100% 
relative humidity the air would hold as much water as it can, so little if any 
transpirational water loss will occur.

Other factors can affect water uptake too, for example, wind.  Plants lose 
more water in windy weather and it still weather.  Maybe you can use a fan 
and do another experiment with leafy celery stalks to seewind effects.

On my web page there are descriptions of some similar plant experiments. Go 
to  and click the BIOLAB button. In 
the section for Elementary Education there are some plant experiments that 
I have constructed for grade school teachers using common materials.