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- 

 duction and a very similar process obtains in all bulbous 

 plants. 



Most perennial plants produce a crop of flowers and 

 fruit 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 to 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 



