222 
Journal of Agricultural Research 
Vol. XIV, No. 6 
“Cell Theory,” clearly set forth the relation between cells, tissues, and 
organs, and also treated in some detail the origin and function of each. 
Other investigators, stimulated by his work, added new facts and cor¬ 
rected the old errors, until finally a fairly clear understanding of the 
general internal anatomy of plants was gained. As may be expected, 
much of this earlier work relates to the xylem, as this tissue is by far 
the most easily distinguished under low magnification. The phloem 
remained a mystery up to the time of Hartig. This investigator reports 
and describes phloem elements, but it remained for Von Mohl to give 
a clear conception of the sieve tube and to show its significance as a 
conducting unit for plastic materials. Von Mohl, and later other investi¬ 
gators, reported the occurrence of sieve tubes in numerous plant families, 
but Hanstein (2) 1 was first to report their occurrence in the Solanaceae, 
the family to which the potato belongs. 
Vesque (5) gives a short discussion on the distribution of the external 
and internal phloem in the Solanaceae. He also reports the occurrence 
of phloem fibers, but states that they are absent in the internal phloem 
when they are wanting in the external region. 
Petersen (9) confirms Vesque and slightly extends his observations 
on the distribution and the relative amount of external and internal 
phloem in the different genera of the Solanaceae. 
The first detailed discussion of the histology of the Solanaceae is given 
by Weiss (10). He writes as follows: 
The internal phloem groups always accompany the leaf-traces into the leaf and are 
differentiated only a little earlier than are the groups of external phloem. A distinct 
cambium is never developed between internal phloem and xylem. The internal 
phloem groups remain distinct and only in the smaller veins of the leaf blade do they 
unite with the external phloem. On the other hand, the internal phloem may be 
considered as derived from the external phloem, a theory which would explain the 
presence in the pith of fibers characteristic of the external phloem. 
In 1872 Jurgens (4) published a thesis on the anatomy and physiology 
of the potato tuber in which he gives a general yet comprehensive ac¬ 
count of the histological structure and development of the tuber as well 
as of the plant itself. Unlike other investigators of his time, he did not 
believe that the periderm is formed from the epidermis, but from the 
subepidermal layer, the original epidermis having become sloughed off. 
The work of Schacht (r) is of interest only on account of its numerous 
and beautiful plates illustrating the internal anatomy and the mor¬ 
phology of the plant and the tuber. 
De Vries, after publishing two papers, one on the development and 
germination of the potato tuber (7), the other on the seed (6), reported 
his researches on the anatomy and the physiology of the potato plant in 
the “ Landwirtschaftliche Jahrbiicher” (<?). This paper is of consider¬ 
able importance, especially for the physiologists. In fact, it is the only 
1 Reference is made by number (italic) to “ Literature cited,” pp. 251-252. 
