4 
The potato, for instance, though it produces flowers in 
this country very rarely contrives to ripen its seeds in our 
climate and can only be propagated in England by its 
tubers, which are indeed the sole reason for its cultivation, 
for these tubers richly stored with food material are of 
the greatest importance as a staple food of mankind. The 
Jerusalem artichoke has the same way of vegetative repro- 
hie and a very similar process obtains in all bulbous 
plants. 
Most perennial plants produce a crop of flowers and 
it every season, in some cases after a shorter or longer 
period of immaturity as is usual for instance in shrubs 
and trees. These flowers may make their appearance in 
spring before the foliage. But in that case the flowers are 
produced at the expense of the food material built up by 
the leaves of the preceding summer, while the fruits are 
generally matured by the activity of the leaves of the same 
season. 
Let us now consider some of the effects of external 
factors upon the growth of plants. If we germinate a seed 
under suitable conditions on the surface of the soil we 
find that when the young root breaks through the seed- 
coat it bends downward and penetrates into the earth. 
That this is due to the effect exerted by gravity on the 
young growing root can be demonstrated by slowly rotat- 
ing a growing plant on a horizontal axis, when it will be 
found’ that the root will grow out horizontally as gravity 
acts first on one side of the root and then on the other, and 
thus its effect is eliminated and the root is not affected. 
The main stem of a plant is equally sensitive to the force 
of gravity, but responds in a different manner growing 
in the opposite direction to it and, if laid down horizon- 
tally, bending upwards at right angles. 
Detailed microscopic examination has shown that 
plants have special regions of perception and it was found 
by Darwin that as regards its sensitiveness to gravity the 
seat of perception was the root-tip. and that if this was 
cut off the root ceased to respond tg gravity. In the stem 
it is known that the perceptive region is not so limited in 
extent. But while the main root and the main stem tend 
by their response to gravity to grow in a vertical direction, 
lateral roots and lateral branches do not respond in a 
similar manner but tend to place themselves more or less 
horizontally or obliquely. A most curious feature of plant 
