1397] FUNDAMENTAL PRINCIPLES OF HEREDITY 3 1 1 



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, &c, 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 



