ZOOLOGY AND BOTANY, MICBOSCOPY^ ETC. 73 



diflferentiated sieve-plate. The volume of this callus-skeleton varies 

 at different times, increasing with age and with the commencement of 

 a period of rest. Under both these conditions the sieve-pores may- 

 become completely stopped ; opening again when the period of active 

 vitality recurs. This description applies both to the sieve-plates on 

 the transverse, and to those on the longitudinal walls of the sieve-tubes. 



The chemical behaviour of the callus does not agree with that of 

 any known group of substances. With acids and alkalies it swells 

 quickly, and is quickly dissolved if they are concentrated ; it is only 

 slightly attacked, or not at all, by ammoniacal oxide of copper. It is 

 not coloured by an alcoholic solution of iodine ; while potassium iodide 

 colours it yellow or brownish yellow ; with the addition of Schultz's 

 solution an intense red-brown colour is produced; this substance 

 alone causes no change of colour, but a considerable swelling. In the 

 polarizing apparatus it remains dark. 



While the sieve-tube is in process of formation, a characteristic 

 change takes place in its contents, in the formation of isolated drops 

 or irregular masses of a colourless or yellowish shining mucilage 

 apparently rich in nitrogen ; these appear in the layer of protoplasm 

 which lines the cavity of the young bast-cells ; and their appearance 

 is followed by the disappearance of the nucleus. These separate 

 masses unite, as a rule, into a parietal band, which is usually many 

 times narrower than the cell. This always remains distinct from the 

 granular protoplasmic parietal layer of the cell itself ; while the fluid 

 contents is known as the " sieve-tube-sap." 



The author was unable to observe more exactly the origin of the 

 sieve-pores and the commencement of the open communication 

 between the separate cells. It probably takes place by the outgrowth 

 of protuberances from the protoplasmic parietal envelope of the sieve- 

 tube, which enter the sieve-pores from opposite sides where two 

 sieve-plates lie opposite to one another, and unite to form connected 

 strings. 



The probable presence was determined, in Lagenaria and Cucurhita, 

 but not in Vitis, of an interior tube, within the parietal protoplasmic 

 layer stretched out between the sieve-plates, and also penetrating the 

 sieve-pores, formed apparently from the substance of the mucilage. 

 The actual perforation of the sieve-pores, doubtful in some cases, was 

 proved in others by the connection between these protoplasmic 

 envelopes of adjacent sieve-tubes. 



The presence of starch-grains in the sieve-tubes was established by 

 the author in the case of Vitis. They are of considerable size, 

 1-5-2 -5 mm. in diameter, and are present when the tubes are still 

 closed, chiefly in the neighbourhood of the septa. Their size precludes 

 the hypothesis that they pass through the sieve-pores. 



In the vine, sieve-tubes occur in the medullary rays, by means of 

 which a communication is kept up between the elements of adjacent 

 bast-rays. They result from the transformation into sieve-tubes of a 

 continuous row of cells of the medullary ray, which penetrate it 

 usually in an oblique direction. Neither these nor the starch-grains 

 were observed in Lagenaria or Cucurhita. 



