32 OUTLINES OF EVOLUTIONARY BIOLOGY 



the form of starch grains, which may afterwards be converted 

 into sugar again and then, in solution, transferred to other parts 

 of the plant, where it is redeposited and stored up, once more in the 

 form of starch grains, for future use, as in the potato and in starch- 

 containing seeds such as peas and beans. 



Both starch and chlorophyll are, at any rate usually, formed in 

 the cell in connection with specialized portions of the protoplasm 

 known as plastids. These are regarded as living bodies which 

 are specially concerned in the formation of chlorophyll, starch and 

 other substances. When they contain chlorophyll they are termed 

 chloroplastids, and in the higher plants they take the form of 

 numerous minute "chlorophyll corpuscles" of definite shape, 

 which occur in abundance in the cells of all green parts, and in 

 connection with which the starch grains are formed (vide Fig. 26). 

 In Haematococcus practically the whole central mass of cytoplasm 

 is coloured by the chlorophyll (or haematochrome) and may 

 perhaps be regarded as a single large chloroplastid. The starch, 

 however, is collected around small, specialized, proteid bodies 

 imbedded in the general mass of cytoplasm. These are known 

 as pyrenoids (Fig. 5, C, py.). 



We have thus seen how the green plant obtains the carbon, 

 hydrogen and oxygen which it requires for the manufacture of 

 protoplasm. Other elements, however, have to be combined with 

 the molecules of carbohydrate before proteids can be formed. These 

 are nitrogen, sulphur and, sometimes at any rate, phosphorus, all 

 of which are obtained by green plants by the decomposition of 

 mineral salts nitrates, phosphates and sulphates which exist in 

 solution in the water or damp soil in which the plant grows. In 

 the higher plants these substances are taken up by the root-hairs 

 and transmitted to the leaves by a system of vessels and tracheids 

 analogous to the circulatory system of animals. In such a plant 

 as Haematococcus they simply diffuse into the protoplasmic body 

 from the surrounding water by the process of osmosis. Exactly 

 what happens when they meet with the carbohydrates we do not 

 know, but further chemical combinations must take place 

 under the influence of sunlight, which finally result in the 

 formation of new proteid molecules which are added to the 

 already existing protoplasm. 



Respiration in Haematococcus probably takes place exactly 

 as in Amoeba, but it is more easily studied in the case of the 

 higher plants. In daylight the process is greatly obscured by 



