Page:The Journal of Indian Botany.djvu/169

339

Harper, R. A. The structure of protoplasm. Am. Jour. Bot. G (1919) pp. 273—300.

A quarter of a century ago there was much speculation regarding the physical structure of protoplasm. It was then expected that better fixation, better staining processes, and more accurate microscopic observation would reveal this structure. Since then there has been much evidence accumulated bearing on the problem, though there has been little attempt to formulate rigid theories of the structure of protoplasm.

Professor Harper of Columbia University, in an address before the Botanical Society of America in 1917, has brought to bear on the problem what he considers to be the most important evidence from recent work in cytology, in the chemistry of the colloidal state, and in genetics. He points out that evolution has been along the lines of specialization of cells groups and division of labour between cells, rather than by any change in funda- mental cell organization.

He considers that the most 'significant evidence from cytology is the re- cognition of differentiated areas within the protoplasm, within which specific processes are carried on. Examples -are elaioplasts or fat-forming bodies, plastids of various kinds, vacuoles, and chromosomes. In many of the lower plants the chloroplasts appear not to have-.a definite limiting membrane like a plasma membrane, but are more or less vaguely restricted regions of the protoplasm impregnated with chlorophyll. With increasing localization of position and specialization in function, these protoplasmic areas finally be- come permanent and self-perpetuating. Chromosomes, for instance, always arise from pre-existing chromosomes by an astonishingly complicated and precise division. He believes that " the bulk of the literature of the plant chondriosome is a mere tabulation of the appearance of variously fixed and colored particles in the cell body with the hope that such bodies may later be found to be specific and fundamentally significant." The concept of localized areas of protoplasm for specific function would include all the organs of the cell, temporary as well as permanent.

Perhaps the most important evidence from recent chemistry of the col- loidal condition is the "recognition of the fact that the units of colloidal systems, especially those of proteins, carbohydrates, etc., are large enough to be distinguishable.at least with our present microscopes ". Protoplasm is a colloidal substance, often thought of as a simple two-phase system with water or some other non-living substance as one of the phases. Butschli considered it to have an alveolar structure, with the living substance forming the continuous walls and the non-living occupying the vacuoles; BEMERINCK and others conceived of protoplasm as made up of granules of living sub- stance distributed through a non-living matrix. Interpreted in terms of a colloidal solution the first theory would make the continuous phase the more important, while the second would make the disperse phase the more