Answer:
They have  helical thickening of lignin in their walls to prevent collapse
Explanation:
the primary function of Xylem and its conducting vessels tracheids is to provide  support and tranport  water and  minerals  in plants.
 Xylem vessels  have layers of dead materials in the walls called LIGNIN.   Lignin is a polymer; a  very strong, hard materials that stop water passage  through it,because it contains dead cells, 
Therefore  the presence of  deposit of these strong  impermeable, ligified  walls  in its  walls  prevent its collapse during mass flow of water of transpiration pull.
 
        
             
        
        
        
Answer:
ALL OF THE ABOVE
Explanation:
Genetic Drift are the changes in allele frequency of a population that result from RANDOM survival or reproduction of individuals with certain characteristics.  Survival or reproduction of those individuals in the face of some environmental change is a matter of LUCK or CHANCE, not because of their phenotype or genotype.
While in Natural selection, the environmental events that affect a population are likely random, but the survival or reproduction of the individuals depends on their phenotypes and genotypes.
Meanwhile, Gene flow is the movement of genes into or out of a population. Low gen flow can lead to low genetic diversity.
Low population which can cause low genetic diversity, poor habitat conditions and habitat loss, road deaths, and commercial development in panther range are constant threats to the Florida Panther's survival. 
All these causes are related and therefore affects the Florida Panther.
 
        
             
        
        
        
You need to know 1) How much kinetic energy it absorbed before it broke.
2) What materials are able to scratch it.
3) Its mass.