i6o 
MORPHOLOGY OF MEMBERS. 
growth that the internodes expand in a plane which also includes the axis of length, and 
thus become leaf-like, as in Ruscus, Xylophylla, &c. 
In leaves the principal growth is usually in a plane which cuts the stem transversely, 
and is mostly symmetrical right and left of a plane which includes the axes of length 
both of the leaf and the stem ; the common form of leaves is therefore that of thin 
plates symmetrically divided in half in the direction of their length. There occur, 
however, cylindrical and roundish tuber-like leaves, in which the growth has been nearly 
equally rapid in all diameters at right angles to the axis of the leaf, as in Mesembryan- 
themum echinatum ^. 
Sect. 22. Hair (Trichome) is the term given in the higher plants to 
those outgrowths which arise only from the epidermis, e. from the layer of cells 
which always remains the outermost in roots, stems, and leaves, whether these 
outgrowths assume the form of simple tubular protuberances, rows or plates of cells, 
or masses of tissue, or have the physiological character of woolly envelopes of the 
young leaves, root-like absorbing organs as in Muscinese, glands, prickles, or spo- 
rangia as in Ferns ^. 
Hairs may originate from the primary meristem of the growing point, or from 
young leaves and lateral shoots, if an external layer of cells has already been 
differentiated as dermatogen, as in Phanerogams; but they may originate also in 
much older parts the tissue-systems of which have already become further differ- 
entiated, and which exhibit interstitial growth, because in such cases the epidermis 
produces new cells, for example stomata, and long remains capable of cell- division. 
When hairs spring from the growing point, they are usually formed after the 
leaves, i. e. further from the apex than the youngest leaves ; but it also occurs in 
Phanerogams that they are developed above the youngest leaves and nearer to the 
apex, the outermost layer of cells of the growing point having in this case already 
become differentiated as dermatogen, as in Uiricularia according to Pringsheim. 
In Muscineae and Vascular Cryptogams also, where the leaves become visible long 
before the differentiation of the external layers of tissue, the hairs do not appear on 
the surface of the stem till a later period and further from the apex. 
If the hairs arise near the apex of a growing point or on a zone of interstitial 
basal growth, as do the sporangia of Hymenophyllaceae, they may be arranged 
according to a definite law, which is not the case with hairs that spring from older 
organs, or at least not evidently so. 
Hairs are always strikingly different in their form from the leaves and lateral 
shoots of the same plant, although they sometimes bear a certain resemblance to 
these organs in other plants. The development in size of a single hair is usually 
extremely small compared to that of the member which produces it ; even the mass 
of all the hairs of a leaf, root, or stem is generally quite inconsiderable compared 
to the weight of the organ. 
^ [The leaf is also frequently unsymmetrical, i.e. the growth has not been equally vigorous 
of the two halves separated by the axial plane, as in the lime, Begonia, &c.] 
^ Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde. Vienna 1871, p. 33. — Compare 
also Sects. 15 and 19 (b). — Warming, Sur la difference entre les trichomes et les epiblastemes, d'un 
ordre plus eleve (extract from the Videnskabelige Meddelelser de la societe d'Hist. Nat. de 
Copenhague, nos. 10-12, 1872. 
2 [Hairs may develope into adventitious buds, as in Begonia; see Caruel, Trasformazione di 
peli in gemme, Nuov. Giorn. Bot. Ital., July 1875.] 
