284 BOTANY PART I 



The new formation of epidermis, which occurs in the normal course of develop- 

 ment in certain Araceae with perforations in their leaves, may be referred to here. 

 In Monster a deliciosa particular limited regions of the laminae of quite young leaves 

 die. Around these spots the mesophyll divides and forms from the outermost 

 layer of cells a secondary epidermis, clothing the perforations and connecting with 

 the primary epidermis of the upper and lower surface of the leaf. In the normal 

 development of plants many processes which can be regarded as regenerative take 

 place such as repeated cork-formation (p. 163). 



In addition to the fact that regeneration occurs, the question as 

 to where this takes place is of interest. The polarity which exists 

 in the intact plant is frequently manifested in regeneration. Thus 

 shoots tend to appear at the apical end and roots at the basal end 

 of portions of stems, while the opposite distribution is found in roots. 

 In more lowly-organised plants polarity is often apparent in the re- 

 generative process, as when each of the single cells separated from a 

 Cladophora forms a colourless rhizoid at the base and a green filament 

 at the apical end. 



This contrast of base and apex does not appear in regeneration 

 from foliage leaves ; this may be connected with the fact that the 

 regenerating leaf is not included in the new formation. Frequently 

 a new plant arises at the base of the leaf, which then dies off. Some- 

 times regeneration proceeds from the general surface of the leaf 

 (Torenia\ but frequently the place of regeneration can be determined 

 by cutting the lamina, the new plants forming above the incisions 

 (Begonia, Fig. 258). 



The phenomena of regeneration have great importance in horticulture, since 

 they allow of plants being rapidly multiplied without the aid of seeds. In 

 artificial reproduction detached pieces of plants are made use of for the purpose of 

 producing fresh complete plants. In many cases this is easily done, but in others 

 it is more difficult or even impossible. The favourite and easiest method is by 

 means of CUTTINGS, that is, the planting of cut branches in water, sand, or earth, in 

 which they take root (Oleander, Pelargonium, Tradescantia, Fuchsia, Willow, etc.). 

 Many plants may be propagated from even a single leaf or portion of a leaf, as, 

 for instance, is usually the case with Begonias. In other cases the leaves, while 

 still on the parent plant, have the power to produce adventitious buds, and in 

 this way give rise to new plants. The Dandelion possesses the capability of 

 developing from small portions of the root, and to this peculiarity is due the 

 difficulty with which it is destroyed. 



(b) Elongation. The meristematic primordia require to enlarge 

 and unfold before they can become functional, and this increase of 

 size is effected in a peculiar and economical fashion. It results 

 mainly from absorption of water from without. Organic material 

 is of course required for the extension of surface of the cell walls, 

 but there is no need of an increase in protoplasm during the enlarge- 

 ment. There is a great difference in this respect between the growth 

 of a plant and a typical animal ; nothing corresponding to this " phase 

 of elongation " is met with in the latter. 



