1397) FUNDAMENTAL PRINCIPLES OF HEREDITY 311 
cork,—this is known as the cork-cambium. We are all of us 
familiar with the little brown scars on plums, &c., that have been 
slightly injured when green: these are due to the local development 
of a layer of embryonic tissue below the injured surface, and the 
formation of a thin protective layer of cork therefrom. 
Colonial propagation in Flowering Plants may take place by the 
separation of buds (which form normally at the growing point), or 
by development of so-called adventitious buds from the embryonic 
cambium zone of the stem or roots. Such propagation by minute 
fragments as occurs in Mosses is unknown here; but larger frag- 
ments of leaves can frequently produce buds and ultimately plants. 
The cells within the cut surfaces produce an embryonic tissue, which 
gives rise both to a protective skin of cork and to adventitious buds. 
The readiness to form cork and adventitious buds in this way 
varies extremely, and with this the power of leaf propagation. For the 
formation of cork is an indispensable protection against the opposite 
dangers of drying up on the one hand, and of the attacks of microbes 
and moulds on the other. Again most Begonias are readily propagated 
by pieces of leaf ; but the bulbous varieties form a mass of embryonic 
tissue, well protected by cork, which remains for months or years 
before active buds are developed, so that they were long thought in- 
capable of this mode of reproduction. Not only Begonias, but Gloxinias 
and other members of the showy order Gesneriaceae, the Peperomias 
with their massive speckled or veined foliage, and Chrysanthemums, 
are habitually multiplied in this way ; and the list of possibilities in 
this direction is daily increasing. 
On reviewing these facts we see that the law of collateral trans- 
mission applies to Plants as well as to Animals, but that they have 
much greater powers of colonial propagation, by the formation of 
embryonic tissue from already specialised colonial cells, and by the 
persistence of a portion of the colony (the growing point, and in 
Exogens the cambium layers) in the embryonic state. The fact that 
green cells can manufacture plant food in the light explains the 
greater vitality and propagative power of small Vegetable fragments 
as compared with those of Animals; and it is needless to assume 
any more recondite intrinsic differences. Even in this mode of 
propagation, the law of collateral transmission holds; for many of 
the cell-forms of plants, such as hairs, wood-cells, &., are abso- 
lutely sterile, and consequently can never take part in the formation 
of an embryonic tissue capable of giving rise to a new plant. 
Thus, throughout the Higher Kingdoms we find the problem of 
heredity rests on different data to those supplied by the Protista. 
In these lowly forms, where the law of direct transmission prevails, 
it is easy to admit that when a cell resolves itself into two new ones 
which exactly reproduce its original state, they should each possess 
