RECENT ADVANCES IN VEGETABLE CYTOLOGY. 35 



form tracheids, vessels and sieve tubes, but that the nucleus 

 as a whole is losing, and still more rapidly, those substances 

 which are capable of being removed by peptic digestion 

 from the cell. The facts seem to suggest that it is albumin, 

 or some other proteid, which is disappearing ; and it is 

 clear that the loss is due to a change in the nucleus itself, 

 irrespective of the amount of nutrition available in the 

 surrounding plasma, since the change is extremely obvious 

 in the degenerating nuclei of sieve tubes, in spite of 

 the fact that they are surrounded by abundant albuminous 

 substances in the slimy contents of the cells. On the other 

 hand, in those cells which are growing in size, preparatory 

 to further divisions, such as in spore-mother-cells, the 

 increase in albuminous substances, both in the nucleus 

 generally, and especially in the nucleolus, is strongly marked. 

 Spore-mother-cells, as a rule, pass through a relatively long 

 period of growth, and hence we might perhaps anticipate 

 (as we find to be the case) that they exaggerate the changes 

 seen in the dividing and growing cells of the apical meri- 

 stem. But I do not wish to lay too much stress on this, 

 because we know that other, and profound, changes occur 

 during the growth of spore-mother-cells, and it is uncertain 

 to what extent the facts just mentioned may be connected 

 with them. 



It may possibly be objected that observations like those 

 of Zacharias are open to adverse criticism on the ground that 

 the chemistry, and a fortiori the microchemistry, of the 

 proteids and other substances which occur in cells is as yet 

 in such an unsatisfactory condition. But this objection is 

 really not a legitimate one. We know that certain struc- 

 tures in the cell are differentiated by their selective action 

 on certain dyes, and it is to this fact that their recognition 

 was due in the first instance. But we find the action of 

 certain solvents to yield no less definite results. Given a 

 nucleus in a particular condition (as judged by the structure 

 rendered visible by staining), and it will be found that the 

 degree of solubility of its constituent substances is charac- 

 teristic for the particular stage in the life history of the cell 

 or of the nucleus which may happen to have been selected. 



