38 BOTANY 



called isomeric chemical compounds. The protoplasmic units are 

 not supposed to be molecules, but are conceived as made up of many 

 molecules, and represent, therefore, not chemical but physical com- 

 plexes. These units are supposed to combine with more and more 

 complicated structures which finally become large enough to be 

 visible by the aid of the microscope. The pangens must be assumed 

 to have the power of growth and division, resembling in this respect 

 the essential organs of the cell, the nucleus and plastids. 



Chemical Composition of Protoplasm 



Protoplasm is in no sense of the word a definite chemical sub- 

 stance like starch or fat, for instance, but is a physical mixture of 

 different units, each of which is in turn made up of excessively com- 

 plicated molecules, principally albuminoid in character. The com- 

 ponent parts of the protoplasm are exceedingly unstable, continual 

 change being a necessary condition for the maintenance of its activity. 

 As a result of this activity there are constantly produced substances 

 which serve either as plastic material for the growth of the proto- 

 plasm, such as starch, sugar, aleurone, etc., or are waste products like 

 resins and crystals. It is not always easy to decide as to the nature 

 of some of these manufactures of the protoplasm which are not 

 always to be distinguished from microsomes which are parts of the 

 active protoplasm. It becomes clear, then, that any definite chemi- 

 cal formula for protoplasm is out of the question, and all analyses 

 are merely approximate. 



Active protoplasm is always saturated with water, which ordinarily 

 constitutes about 75% of its weight, sometimes amounting to 95% 

 in delicate aquatic plants. A large part of the water may be ex- 

 tracted by drying, and the residue, on analysis, always reveals cer- 

 tain chemical elements which are never absent, and which can be 

 shown to be essential for the building up of the protoplasm. Other 

 elements are also usually present, but may be absent in many cases. 

 The most important components of the proteids which form the 

 basis of the protoplasmic structures are Oxygen, Hydrogen, Carbon, 

 and Nitrogen. Sulphur and Phosphorus are also probably essential 

 constituents of protoplasm, and for the normal growth of green 

 plants, Potassium, Calcium, Magnesium, and Iron are necessary. 

 These elements may be combined in an infinite variety of ways, 

 many of Avhich have been artificially produced, but most of which 

 have not as yet yielded to the tests of the laboratory. 



A considerable number of other elements are sometimes found, 

 but are not present in all plants. Thus in the large Kelps, Iodine 

 and Bromine are present, and Silicon is a very common element in 

 many land plants, such as the Grasses, Horsetails, and many others. 



