j ANIMALS AND PLANTS 39 



though never wholly absent. Intracellular movements of the 

 protoplasm are, however, found in all Plants. 



In Plants we find no distinct nervous system formed of cells 

 and differentiated from other tissues with centres and branches and 

 sense-organs. These are more or less obvious in all Metazoa, 

 traces being even found in the Sponges. 



We may then define Plants as beings which have the power 

 of manufacturing true food-stuffs from lower chemical substances 

 than proteids, often with the absorption of energy. They have 

 the power of surrounding themselves with a cell-wall, usually of 

 cellulose, and of growing and dividing freely in this state, in 

 which animal-like changes of form and locomotion are impossible ; 

 their colonies are almost always fixed or floating ; free locomotion 

 is only possible in the case of their naked reproductive cells, 

 and is transitory even in these. The movements of motile parts 

 of complex plant-organisms are due to the changes in the osmotic 

 powers of cells as a whole, and not to the contraction of 

 differentiated fibrils in the cytoplasm of individual cells. Plants 

 that can form carbohydrates with liberation of free oxygen have 

 always definite plastids coloured with a lipochrome 1 pigment, 

 or else (in the Phycochromaceae) the whole plasma is so coloured. 

 Solid food is never taken into the free plant- cell nor into an 

 internal cavity in complex Plants. If, as in insectivorous Plants, 

 it is digested and absorbed, it is always in contact with the 

 morphological external surface. In the complex Plants apocytes 

 and syncytes are rare the cells being each invested with its own 

 wall, and, at most, only communicating by minute threads with 

 its neighbours. No trace of a central nervous system with 

 differentiated connexions can be made out. 



Animals all require proteid food ; their cyst-walls are never 

 formed of cellulose; their cells usually divide in the naked 

 condition . only, or if encysted, no secondary party-walls are 

 formed between the daughter-cells to unite them into a vegetative 

 colony. Their colonies are usually locomotive, or, if fixed, their 

 parts largely retain their powers of relative motion, and are often 

 provided on their free surfaces with cilia or flagella ; and many 

 cells have differentiated in their cytoplasm contractile muscular 

 fibrils. Their food (except in a few parasitic groups) is always taken 



1 Pigments soluble in the ordinary solvents of fats, such as ether benzol, 

 chloroform, etc. 



