ON CERTAIN LEAF-HAIR STRUCTURES. 257 
Probably the best example of the mealy Primulas is P. 
farinosa. Its glandsare short two-celled and much resemble 
those from Notholena Hookert. The secretion consists of 
short rod-like bodies that are readily soluble in alcohol and 
ether. These solutions leave on evaporation light yellow 
sharply-pointed crystals that aggregate into characteristic 
stellate masses. They certainly differ from the ceroptene 
crystals, which assume bluntly-terminated prismatic or tab- 
ular forms and whose aggregates approach in some measure 
the fern-leaved type. This contrast may be seen by com- 
parison of fig. 4 with fig. 5. 
The probable function of these glands is a matter that 
might give rise to considerable speculation. In examining 
the various forms which they assume one cannot but be 
impressed with the idea that they are a specific organ of the 
plant formed for some definite purpose. It does not seem 
reasonable to suppose that these secretions are mere waste 
products necessarily produced in the complicated processes 
of cellular growth. 
The only plausible function which I can assign to them 
is that of forming a protection for both the young spores and 
for the epidermis itself against excessive moisture and heat. 
The position and chemical nature of the indument is such 
as to render it impossible for water to reach the lower leaf 
surface or the spores that are surrounded by this farinose 
material. Another fact to be considered is that nearly all 
those species of Notholena, Gymnogramme and Cheilanthes 
that do not bear these glands are provided with a thick 
growth of hairs or scales, whose function is almost univer- 
sally admitted to be that of protection. Furthermore, in 
looking over the habitats of these groups of plants it is seen 
that a large proportion are natives of the arid regions, that 
is of countries in which protective devices against the fierce 
heat of the sun during a portion of the year, and against a 
superabundance of moisture during the remainder, are by no 
means uncommon, 
