K.—BOTANY. 199 
the deposition of encrusting substances on the parts which persisted. Von 
Mohl correctly described simple pits but curiously enough did not 
understand bordered pits which were first elucidated by Schacht. Von 
Mohl gave the first correct account of stomates; he observed their 
movements which he associated with variations in the turgidity of the 
guard cells which he correlated with the accumulation of soluble 
assimilatory products due to the activity of the chloroplasts. His 
observations on periderm and cambial activity are of first-rate importance. 
Von Mohl’s technical ability and power of observation were so great that 
it is, perhaps, remarkable that he did not do more; thus some of his 
preparations, which I am told are still in existence, show the continuity 
of protoplasm ; he did not describe it, did he observe it? He certainly 
never,agreed with Hartig that the sieve plate was perforated. But Von 
Mohl was essentially a practical man, slow to draw a conclusion and 
suspicious of speculation; his work however, was complemented by a 
man of greater imagination: Nageli. Nageli, famous in all branches 
of botany, traced the differentiation of the desmogen strands into primary 
vascular tissues and correlated phyllotaxis with nodal and internodal 
organisation. He investigated the structure of sieve tubes and elucidated 
the anomalous secondary thickening in angiosperms. He systematised 
anatomical knowledge and this led to progress, for it is to be remembered 
that knowledge is advanced by its periodic ordering. His investigations 
on the cell wall and on the starch grain are classical: the recognition of 
the cell wall as a product of protoplasmic activity, the striation of the 
cell wall, growth by intussusception and the occurrence of granulose and 
amylose in the starch grain, need only be mentioned. The results he 
obtained by the use of polarised light were as important as those of 
Sponsler and others by the use of X-rays in our own times. 
The investigation of von Mohl and Nageli were expanded in various 
directions by Bureau, Criiger, Hanstein, Radlkofer, and more especially 
Sanio, Th. Hartig and van Tieghem. 
Cytology.—Although Robert Brown discovered the nucleus a hundred 
years ago, fifty years elapsed before its structure and changes were 
critically studied. This is hardly surprising since the new learning led 
men to the pursuit of the more obvious; to phytotomy, to use an old 
word, and to life-histories. Further, cytology had to await the arrival 
of new methods of fixation and staining and some attainment of the 
knowledge of the colloidal state; also the microtome, as we know it, 
had to be evolved. In the earlier study of cytology, von Mohl and 
Niageli were the pioneers. Von Mohl was the first to see cell division but, 
apparently, he did not appreciate the significance of his observations. 
Later, he studied the ‘ mucilage’ described by Schleiden in 1838; he 
recognised it as a distinct component of the cell and gave it its present 
name, protoplasm. He described the primordial utricle and realised that 
the streaming movements of the cytoplasm were independent of the cell 
sap. Further, von Mohl perceived that the matrix of the chloroplast was 
protoplasmic. Niageli more carefully observed the nucleus and appreciated 
the facts of cell division more truly; and in this he was the first to 
recognise that the nucleus is the starting-point. Later, in 1855, Unger 
suggested the identity of the protoplasm of the vegetable with the sarcode 
