How Buds on Trees Survive the Cold (and Related Mysteries) 

Ben Stark                      Illinois Institute of Technology
                               IIT Center
                               Chicago IL 60616
                               312-567-3488

Objectives:

This science "mystery" was originally presented to second and third graders, 
but is probably appropriate for grades 2-6.  The objectives are (1) to see how 
the delicate tissues of a tree bud are insulated so that they can survive the 
winter, (2) to learn how to determine the age of a branch/twig, and (3) to see 
how growth of branches can vary from year to year.

Materials Needed:

Several small branches/twigs, taken from, preferably, a horse chestnut (buckeye) 
tree in late Fall or Winter (after the leaves have all fallen). 

razor blades (for younger children, to be used by teacher only)

hand lenses

Strategy:

This lesson makes a perfect "science mystery."  One way to introduce it is to 
ask the children how they stay warm in the Winter (they stay inside and their 
houses are heated, they wear warm clothes, they sleep under blankets, etc.).  
Then you show a branch and focus on the "terminal buds" on that branch (the 
terminal buds are those at the end of each branch).  You point out that the 
terminal bud contains a baby stem and baby leaves and remind them that these 
"babies" have to stay warm all Winter also.  The mystery is, how can they stay 
warm without the advantages the children have?  You might remind the children 
also how cold it has gotten outside that winter. 

Then show the children the outside of the bud(s).  Note that the buds are 
covered by a series of hard overlapping "bud scales."  Then, using a razor 
blade, slice open one or more buds laterally (left to right) through their 
centers.  Observe with the aid of the hand lenses (or with the naked eye if no 
lenses are available) and perhaps make a drawing of what is seen. 

Things to notice in particular are the many layers of the bud scales on the 
outside of the bud, the "baby" (embryonic) shoot and leaves in the center of the 
bud (which might be a bit hard to see), and the "cottony" packing material which 
tightly surrounds the shoot/leaves underneath the layers of the bud scales.  The 
answer to the mystery, then, is that the embryonic shoot and leaves are 
protected from the cold by both the layers of bud scales and the cottony 
material, and so are able to survive the Winter cold. 

In the Spring, the embryonic shoot/leaves in each bud begin to grow.  As they 
do, they shed the cottony material and the bud scales, which fall off the 
branch.  Loss of the latter produces a series of bud scale scars at the base of 
the former bud; these look like a series of (mostly) parallel lines around the 
circumference of the branch.  The position of each group of bud scale scars, 
then, permanently records the position of a terminal bud that had existed at one 
time in that position on that branch.  The distance between adjacent bud scale 
scars on a branch represents the growth of that branch in one growing season 
(this might open a lively discussion by determining if the growth of a branch 
had been the same in each year and, if not, what variables--heat, rainfall, 
etc.--might have led to growth variations). The overall age of the branch can be 
determined by counting the number of groups of bud scale scars on it.  Note that 
if your branch has smaller branches growing out of it, they will have to be 
younger than the main branch, and this should be confirmed by the bud scale 
patterns on each.    

Performance Assessment:

Students could be graded on their diagrams of the dissected buds, with points 
given for accuracy, neatness, detail, and correct labelling.  You could also ask 
students for a short written report covering such topics as the age of their 
branches (and how they determined them), if there was variable growth from year 
to year in the branch and how this was determined, etc. 

References:

Probably any good high school or college general biology or botany text would 
be a useful resource for this activity.