32 Plant Physiology 



are a constant environmental factor, it is safe to assume 

 that the organism is adjusted to this factor, so that it 

 manifests what is known as a tonic response. 



Another case of response has already been noted : under 

 different intensities of light the orientation of the chloro- 

 plasts may be diverse. In the cells of the duckweed 

 (favorable for observation) the chloroplasts are distributed 

 in the upper portion, or dome, of the cell and also across 

 the bottom, in diffused light; while in bright light they 

 lie at the sides and one above another. The position in 

 the dark is along the vertical walls, also the horizontal 

 wall when that does not abut upon the epidermis. This 

 type of response to light (in this instance) is generally 

 regarded as denoting protoplasmic irritability. 



LABORATORY WORK 



Living cells. Remove with the forceps or scissors the fila- 

 mentous, purplish hairs from the stamens of any available 

 species of Tradescantia. Mount these, and note carefully the 

 form and size of the distinct cells. Distinguish cell-wall, pro- 

 toplasm, and colored vacuole, and observe each of these crit- 

 ically. Describe the peripheral and strand cytoplasm, also the 

 form and position of the nucleus, with nucleolus. Draw. Com- 

 pare the cell drawn with others both nearer the base and the 

 apex of the filament. Kill with tincture of iodine, and examine. 



As in the preceding, study the cells of hairs clipped from a 

 petiole of a fairly young squash or pumpkin leaf. In this case 

 note also the form and position of the plastids (chloroplasts). 

 Peel off a little of the epidermis of Cyclamen or Begonia ; 

 mount, study, and describe these cells. For comparison study 

 and draw a stained preparation of a root-tip or bud-apex and 

 compare with the previous material. 



