Bean grown
     

Why to study a plant growth

  1. The plants are the easier living beings to be studied, because they are anywhere and we can observe them without problems.
  2. It is easy to observe how a plant grows: their vital cycle is continuous and  often fast.
  3. To grow a plant is a motivational approach to biology and science in general, because children like to do it.
  4. The structure of a plant is simpler to be study than the structure of an animal, because we can observe it from the outside and it is easier to examine vegetal cells than an animal cells at under a microscope
  5. With the plants it is possible to do many experiments, developing the scientific way of thinking

    Growing a plan is a good way to study mathematics: we can analyse data statistically, plot and compare graphs…

     

  

What happens when a bean grows

If the environmental conditions are favourable, the seed germinates, i.e. the embryo becomes a little plant.

The seed coat (integument) breaks, the radicle goes out  from the seed and lengthens into the soil; the stem lengthens and  go out from the soil  along with the cotyledons ( fleshy and rich in nutrients leaves), then the first little leaves appear.

When the plant becomes able to make food by means of photosynthesis, the cotyledons shrivel and fall.

  

Diary

7th February 2009: the dry kidney beans are soaked for 24 hours, then put on a plate between many layers of wet Scottex paper, till the  radicle appears.


10th February 2009: every seeds germinates and is planted in the soil. We plante five  beans in five equal plastic pots containing the same amount of soil but kept under different conditions:

  • The pot # 1 iskept inside the classroom: temperature of 20°C, in the light
  • The pot # 2 is  kept in the dark inside a cabinet in the classroom
  • The pot # 3 iskept inside the classroom: temperature of 20°C, in the light, fertilizer (NPK 15-10-30)
  • The pot # 4 iskept outside at a low temperature (between 2°C and 12°C, in the light)

Each pot is watered with the same amount of water every 2-3 days.

13th February: all the little plants in the pots #1,#2 and #3 appear after two- three days in the soil and after other two days were completely out of the soil.


16th February: all the little plants in the pots #1,#2 and #3 are completely  out of the soil. Nothing happens in the pot # 4.


18th February: we begin to measure the stem length.  The plants in the pot #3 (with fertilizer) are much higher than the plants in the other pots; the height of the plants in the pots #1 (in the light) and #2 (in the dark) is almost the same


26th February: the growth of the plants in the pots # 3 (with fertilizer) slow down and follow the growth rate of the plant in the pot # 1 (without fertilizer). Now the plants in the pot #2 (in the dark) are the highest, but their colour is pale, yellow-green, and the stem is very slim.  


2nd March: the shrivelled cotyledons have fallen 


9th March: the little plants in the pots #4 (low temperature) appear.


11th March: we begin to measure the plants in the pot # 4 (low temperature)


13th March: the plants in the pot #2 (in the dark) are dead


16th March: : the plants in the pot # 4 (low temperature) have doubled their length


20th March: the growth of the plants in the pots #1, #2 and #3 is now very slow. We stop to measure.

The measures in the graphs are the average height of the 5 plants in each pot

Interpretation

All the plants grow following a characteristic behaviour: at the beginning the growth is fast, then it slows down. The graph which describes this behaviour is a S shaped curve.

How different factors (e.g. water or fertilizer amount)  influence the shape of the curve?

  • The temperature is a very important factor. The seeds germinate only between 15°C and 25°C (approximately) and when they are planted in the soil, they need at least 8°C- 12°C to develop a visible plant. 
  • The fertilizer accelerates the growth of the plant only at the beginning, it seems to give a “starting push”, but then it doesn’t affect the growth.  
  • The light is necessary for the plant growth, when it starts to photosynthesize, but for weeks a plant can live also in the dark, consuming the nutrients stored in the cotyledons.  The high stem developed by the plants in the dark and their fragile appearance is a consequence of the adaptation to the environment.

     This is the explanation:
In nature a plant germinate in the dark if the seed is very deep in the soil or under a stone. Under this adverse conditions, the plant concentrates all its efforts on reaching  the light as soon as possible. The stem grows very much in length, but not in thickness. The leaves remain little and yellow: the chlorophyll isn’t synthesized because without light the plant doesn’t need it! The plant may succeed in its efforts to go out into the light and survive, or die: it is a bet!