224 BOTANICAL GAZETTE [SEPTEMBER 
3. GOEBEL, K., Einleitung in die experimentelle Morphologie der Pflanzen. 
tans Teu ner. 1908. 
. McCattum, W. B., On the nature of stimulus causing the change of form - 
ie situcbure in Proserpinaca ee Bor. Gaz. 34:93-108. figs. 10. 
5. MacticuGas, sg eS Practical textbook of plant physiology. New York: 
Longmans. Igor. 
EXPLANATION OF PLATES II-IV © 
PLATE II 
iildssbow triticoides Walsh < on stem of Salix cordata Muhl.: 
aeiee section showing general arrangement of larval chambers and distri- 
bution of — tissue throughout the cortex and pith of the gall; X1o. 
Fic. abdophaga triticoides Walsh on stem of Salix cordata Mubl.: 
hemlecon ‘eeckion in which is shown the typical character of the aeriferous 
tissue produced under the stimulus of the gall producer; X50. 
Fic. 3.—Phytophaga rigidae O.S. on Salix discolor Muhl.: a stem gap 
which shows aeriferous tissue produced in excess of normal amount; X90. 
Fic. 4.—Rhabdophaga strobiloides Walsh on stem of Salix cordata Muhl.: 
a longitudinal section showing aeriferous tissue produced in pith of gall axis; _ 
X25. 
PLATE III 
Fic. 5.—Pontania pomum Walsh on Salix cordata Muhl.: section of a 
nearly full-grown gall in which aeriferous tissue is produced throughout its 
mass; X18 
Fic. 6.—Phytophaga rigidae O.S. on Salix discolor Muhl.: gall chamber 
includes all the pith area of the stem; aeriferous tissue of the characteristic 
type occupies whole of cortical region of the gall; x10 
Fic. 7.—Populus balsamifera L.: transverse sectiol of stem pith in which 
aeriferous tissue is normally present; X 8o. 
- Fic. 8.—Salix humilis Marsh.: section of normal stem at level of a stem 
gap; it illustrates the compact nature of the Salix pith and the differentiation 
of aeriferous tissue in the stem gaps; X65. 
PLATE IV 
Fic. 9.—Salix lucida Muhl.: section through base of a normal stem, show- 
ing aeriferous tissue produced in the pith; 
Fic. 10.—Salix lucida Muhl.: transverse bection of normal stem a short 
distance behind the growing point; aeriferous tissue constitutes the entire cor- 
tex, but is completely absent from the pith; this compact nature of the pith is 
typical for all species of Salix except in region of bases of branches as illus- 
trated in preceding figure; X12. 
