PLANT PHYSIOLOGY 153 
time. Leaves of some insectivorous plants also show 
movement in response to contact. 
137. Adaptations to Environment.—Every gardener 
knows how rapid is the growth of weeds in any 
neglected piece of ground and how difficult they are to 
exterminate. When he plants out his flowers he has to 
shade them and water them carefully every day, till 
they are established, while all around in the same soil 
the weeds spring up and flourish, and have to be con- 
tinually rooted up. Why isthis? The weeds obviously 
are better adapted to the existing conditions than the 
cultivated plants; they are either the natural wild 
plants of that particular district, or they may be im- 
ported plants which find their new surroundings exactly 
suited to their requirements, as, for instance, the 
Cosmos, and the Khaki Weed (Tagetes) which are 
spreading so rapidly through the Transvaal. Now we 
know that most flowering plants produce a very large 
number of seeds, and as it is impossible that all these 
can become mature plants, only the fittest will survive. 
Tf all the seedlings were exactly alike, it would be mere 
chance which of them survived in the struggle for ex- 
istence. But they are not all alike, they exhibit 
individual differences, and if any of these “ variations ”’ 
gives them an advantage over the others, they will 
survive, and the new variation will tend to be trans- 
mitted and ultimately to become fixed. 
Examine the common weeds found in any garden and 
notice what special characteristics they have that enable 
them to gain the upper hand. Some have very deep 
tap roots, and a spreading rosette of leaves which covers 
