146 



of dwarf plants. Yet, with equal vegetative conditions, seed from normal 

 plants produces more vigorous specimens than that from dwarfed plants. 



The case of nanism due to scarcity of nutritive substances, which 

 MoUer studied, is not rare in sandy soils. The lack of nitrogen plays the 

 chief part here. This nanism is usually characterized by the fact that, be- 

 sides the general reduction, the relations of the separately produced organs 

 have been changed. In proportion to the whole growth, the root undergoes 

 a greater distention ; but the sex organs suffer a greater retrogression. The 

 number of blossom eyes is ver}^ small. Instead of a cluster or a head, there 

 is often only a single blossom. Where a greater number of blossoms are 

 formed single seeds develop which can germinate. It is easy to understand 

 that the leaf-forms are simplified. 



In discussing dwarf growth, the phenomena of bud variation must be 

 considered. These have no connection with soil conditions or other external 

 vegetative factors. The form of growth up to this time is so changed by 

 some impulse or stimulus, acting temporarily or persistently, that the organic 

 substance is used up in the form of more numerous, shorter, usually thicker, 

 short-leaved branches instead of fewer slender, large-leaved ones, in this 

 way producing witches-brooms. In many cases the incitement to such a 

 changed direction of growth may be found in parasitic attacks. The fungus 

 genus Taphrina (Exoascus) especially irritates the branches of various 

 deciduous trees resulting in the formation of witches-broonis (see Volume 

 II. page 179). In other cases we find rust fungi or mites of the genus 

 Phytoptus. Besides these forms due to parasites, however, some surely 

 exist in which other organisms are not" active. We find especially in her- 

 baceous, quickly growing plants (Campanula, Pelargonium) the occurrence 

 of a bud disease (Polycladia) as a correlation-phenomenon. 



In sickness or loss of blossoming branches, small fleshy bunches are 

 formed, at times, at the base of the stem, made up of closely set bud-eyes, 

 some of which grow out into sickly branches. In diseased thickets growth 

 is often exhausted by a continued new formation of short branches, because 

 the blossoming axes no longer lengthen, but stop growing and turn yellow. 

 In Calluna vulgaris, instead of long blossoming branches, we find blossomless 

 bunches of twigs, pyramidal in form, which might also be called witches- 

 brooms. 



In other cases polycladia and bushy forms are produced by the develop- 

 ment of normally formed but still dormant lateral eyes, when the buds of 

 the tips have been injured. This takes place when wild growths choke out 

 cultivated ones. In conifers, the heart buds grow out and form bushy 

 crowns, which are called "rosette-groivths." The so-called "cow-bushes" — 

 due to injury to beeches, alders, etc.. from the grazing of cattle, are similarly 

 explained. 



Pure bud variations are numerous. In them the growth in length of 

 the individual branches is restricted without any recognizable cause, result- 

 ing in a greater and more rapid development of lateral branches. Among 



