396 
POPULAE SCIENCE KEVIEW. 
with another individual or with suitable food.* And here I 
think we may find some explanation of the fact that there is 
greater simplicity in the movements observed in those plants 
whose protoplasm is not imprisoned in cellulose than in the 
motions of the lowest animals. 
The food of the plant is carbonic acid and ammonia dissolved 
in water ; that of the animal is formed material, solid or viscous 
particles of proteinaceous matters. Hence, while the most 
rhythmic movements are sufficient to bring the plant in con- 
tact with fresh currents of water charged with its nutrition, 
this nutrition is ever present to act as the exciting cause of 
motion. Not so with the more highly organized protoplasm or 
sarcode of the lowest animal. Its food is not everywhere held 
in solution, but is suspended in the form of solid particles,, 
or localized nutritive fluids, hence greater irritability and 
more varied power of locomotion are needed : contact 
with minute particles of varying chemical nature, and the 
irritation of different gases in solution, act variously on the 
more sensitive protoplasm of the animal as exciting causes of 
motion. In the animal kingdom the protoplasm becomes 
more and more differentiated as we ascend the scale, until we 
get distinct sensory and motor systems, nerves, and muscles. 
In the plant, as we ascend the scale, the protoplasm is more 
and more enclosed by its sheath of cellulose, and movements 
become only the rare exception, or the simple results of growth. 
Before proceeding to the consideration of the movements of 
the Diatomacese, let us see what other plants exhibit move- 
ments, and of what kind. The most common movement in 
the higher plants is that resulting from growth, such as the 
underground extension of bulbilli in Orchids, and the elevation 
of trees from the ground by the resistance offered to the- 
downward growth of their roots. Light acts as a great cause 
of movements of a certain kind, the sunflower's daily turning,, 
and the shutting and opening of the petals and leaves of nearly 
all flowers, showing that the light and heat of the sun produce 
certain chemical and physical effects in the tissues of plants 
which result in movement. Similar to these movements but 
dependent on the more accidental irritation of contact with a 
foreign body, are the phenomena exhibited by the stamina of 
the Barbary, by the pollinia of the Orchids, and the leaves of 
many so-called sensitive plants. The sudden extension of the 
spiral flower stalk of Valisneria is a remarkable example of the 
subservience of the phenomenon of motion to reproduction, 
as in the case of the Orchids and others. The active gyrations 
* By food is here meant all external agents necessary to the support 
of life. 
