434 Morphogenetic Factors 



identical pattern of granular cytoplasmic strands on which the thickenings 

 are laid down (p. 193). Other markings in the cell wall have also been 

 traced to cytoplasmic differences. 



The cytoplasm may be concerned in the development of characters 

 above the cellular level. In the cells of air roots of orchids, the bands 

 of wall thickening ( p. 201 ) that keep these cells from collapsing are laid 

 down by the cytoplasm in each cell. They are not isolated structures in 

 single cells, however, but a thickening in one cell is directly adjacent to 

 one in the next, so that a continuous system is produced extending from 

 cell to cell and forming a histological pattern over a considerable mass of 

 tissue. The differential fiber patterns in Luff a (p. 197) and similar cases 

 doubtless originate in the cytoplasm. 



The distribution and configuration of the cytoplasm probably have a 

 more deeply seated relation to development, however, than in these ex- 

 amples of cellular patterns. The plane in which a cell divides, at least 

 in vacuolate cells, is foreshadowed by the orientation of a cytoplasmic 

 plate some time before the axis of the spindle is established (p. 25), a 

 fact which suggests that cell polarity, and thus the direction of growth 

 and ultimately organic form itself, may have their immediate basis in the 

 distribution and patterning of the cytoplasmic body. 



The relation of cytoplasm to such differences in form may be well 

 seen in the coenocytic bodies of many red algae, notably the genus 

 Caulerpa. Here there are no cellular barriers to the passage of cytoplasm 

 from one part of the plant to another, and much streaming takes place. 

 Differences may be observed in various parts of the plant body as to the 

 character of the cytoplasm, part of which is fixed to the inner wall and 

 does not stream. In such plants the differential distribution of the cyto- 

 plasm seems to be related to the differentiation of the plant body, though 

 the mechanisms involved are unknown. The disadvantage in such or- 

 ganisms and the probable reason why they have never been able to de- 

 velop very highly differentiated bodies is their difficulty in keeping the 

 various components of their living material sufficiently isolated so that 

 physiological differences can be maintained effectively and a high degree 

 of organization thus made possible. 



Even in multicellular plants visible differences in distribution of cyto- 

 plasm are related to differentiation. This is especially evident in cases of 

 unequal cell division, as in the formation of trichoblasts in many roots 

 (p. 190). In the mother cell, which is to divide unequally to form a 

 small trichoblast and a larger hairless cell, much of the cytoplasm (and 

 the nucleus) moves toward the end at which the trichoblast will be cut 

 off, so that even before division there is a difference in cytoplasmic dis- 

 tribution. The formation of the new wall finally separates two regions 

 which had already become cytoplasmically different. In the formation 



