THE STRUCTURAL COMPONENTS OF PROTOPLASTS 25 



The problem of membranes involves a further special differentiation 

 of the cytoplasm known as kinoplasm. This has been studied chiefly 

 ill plant cells. It appears typically in the form of strands or channels of 

 fluid streaming with included granules through the unmodified cytoplasm, 

 or frophoplas7n. It seems to form a more or less continuous system with 

 the plasma membrane, the vacuole membrane, and sheaths of similar 

 material about the plastids and nucleus. Like the membranes, the kino- 

 plasmic strands appear to have a high lipide content. With a few impor- 

 tant exceptions, cytologists have neglected the kinoplasm, and it is much 

 in need of further study, especially in view of its apparent relation to the 

 i:)rotoplasmic surface membranes. 



Nucleus. — The nucleus has claimed a large share of the attention of 

 cytologists ever since its discovery more than a century ago. There arc 

 numerous reasons for this. Excepting chloroplasts of most plants, the 

 nucleus is the most conspicuous organ of a protoplast under the microscope, 

 especially in stained preparations. By observing the effects of its removal 

 from certain cells, it has been shown to be necessary for synthetic metab- 

 olism in the protoplast. At the time of division it passes through an 

 amazingly complicated but very orderly series of changes that never fail 

 to fascinate the observer. C>i:ogenetic studies have shown that the 

 mode of ontogenetic development and, hence, the particular characters 

 exhibited by the organism are related to the constitution of the nucleus 

 and, furthermore, that the inheritance or noninheritance of certain 

 parental characters is due to the behavior of the chief nuclear components, 

 the chi'omosomes, during the reproductive stages of the life cycle. Pre- 

 occupation with nuclear behavior has doubtless been too great at times, 

 but fortunately there have been other workers who have stressed the 

 importance of membranes, plastids, and other components of the cyto- 

 some, so that altogether a fairly well balanced conception of the proto- 

 plast's activity is being built up. 



It cannot yet be said that nuclei are present in all animals and plants. 

 In some minute organisms, particularly certain bacteria, there are bodies 

 whose reactions and behavior at least suggest their nuclear nature, but 

 their minute size and various uncertainties regarding the value of the 

 criteria used to distinguish them from other minute bodies leave the ques- 

 tion quite open. In higher organisms it is not so difficult to characterize 

 the nucleus, for in spite of many variations, some of them rather extreme, 

 the same general type of fundamental structure seems to be present in 

 practically all groups. Such typical nuclei will serve as the basis of the 

 following description. 



The nucleus, when not undergoing division, is said to be in the meta- 

 bolic or energic stage because many of its most important functions are 

 exercised at this time. Unfortunately it has long been called the " resting 



