274 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The First Type. 
Trichomes of the first type are characteristic of the Dicotyledons, 
of some divisions of the Monocotyledons, and of the Filices. In 
these groups, as a rule, any cell of the piliferous layer may, at a cer¬ 
tain stage in the development of the tissue, acquire the character of 
a trichome 1 by putting out a hair. All the cells over considerable 
areas may be piliferous, or only a part of them, determined appar¬ 
ently by chance. From the appearance of the cells of the young 
epidermis it is not possible to predict which ones are to become 
trichomes. Below the region of the youngest hairs there is no visi¬ 
ble specialization or predetermination of the future trichomic cells. 
Every cell, we may suppose, is a potential trichome. The actual 
fate of any cell, whether it shall be that of a trichome or of an 
atrichomic element, is first to be ascertained in the young fixed tis¬ 
sue, after the cells have undergone a considerable extension, and 
after cell division has altogether ceased. The first indication of 
trichomic development is the appearance of a papilla, usually near 
the lower or distal end of the epidermal cell, that is, near the end 
lying toward the apex of the root. 
Distribution of the first type in vascular Cryptogams and Angio- 
sperms. — The following groups have been investigated with respect 
to the species named. The trichomes in these species are not prede¬ 
termined in the nascent epidermis of the root, so far as can be seen. 
Filices. 
Hymenophyllaceae : Trichomanes sp. 
Cyatheaceae : Alsopliila australis , A. Cooperi , Cibotium Schiedei , 
Cyathea insignis , C. medullaris, C. regalis, Dicks onia antarc- 
tica , D. davallioides. 
Polypodiaceae : Adiantum aneitense , A. Ccipillus- Veneris , A. cunea- 
tum, A. decorum, A. gracillimum , A. macrophyllum, A. Wie- 
1 It seems necessary, in describing the structures under consideration, to use 
the word trichome for the whole cell, including the tube, or hair, or root-hair, 
and the basal portion imbedded in the epidermis, or base. The adjective tri¬ 
chomic and its opposite atrichomic have been coined for convenience in the 
present discussion. 
