THE MENDELIAN PHENOMENA 13 



not the same as the difference between a tall and a 

 dwarf man. In a human dwarf everything is on a 

 smaller scale than in the normal man. But a dwarf 

 pea is not simply a miniature edition, as it were, of 

 a tall one : it differs from a tall pea in one single 

 characteristic, the length of the internodes, i.e. the 

 sections of the stem between two nodes, or joints, 

 where the leaves are given off. This is shown in 

 Fig. 6, which represents a seedling of a tall, and a 

 seedling of a dwarf pea sown on the same day. The 

 number of nodes between the seed and the point 

 of insertion of the two leaves which are face to face 

 and enclose the growing point of the stem, is the 

 same in both cases, namely five. They can all be 

 clearly seen in the tall seedling, but the lowest one 

 in the dwarf is difficult to make out on account of the 

 shadow. The shortness of the dwarf pea is thus seen 

 to be due solely to the shortness of its internodes. 



If seed is saved from a dwarf plant it will give 

 rise, when sown, to dwarf plants only ; and seed 

 saved from tall plants will only produce tall plants. 

 In other words, both tall and dwarf peas, when left 

 to themselves, breed true. 



It is desirable to state here that the pea is normally 

 self-fertilised. The male and female organs and pro- 

 ducts are both present in every flower ; and what 

 happens when a plant is " left to itself " is that the 

 female products (or egg-cells) of a single flower are 

 fertilised by the male products (the pollen grains) of 

 the same flower. When one kind, such as tall, is 

 crossed with another kind, such as dwarf, the pollen 



