NOTES ON ROOT-HAIRS AND ROOT GROWTH. 
175 
It seems probable that some plants rarely if ever possess, 
root-hairs, that in certain other cases they are formed only under 
certain conditions, and that they are caducous, so that they may 
be found at certain times and seasons, while in others they 
may not come under notice. So far as my own observations 
go, plants whose roots are wholly immersed in water are 
frequently, but by no means universally, destitute of con¬ 
spicuous root-hairs— e.g., Callitriche, Nymphcea, Nasturtium. 
Plants with thick fleshy and woody roots still more rarely 
produce these outgrowths; thus, in the case of the roots 
descending from the corms of a Crocus, the bulbs of a Tulip 
or a Hyacinth, or of the fleshy root fibres of Ranunculus 
Ficaria, &c., there are no conspicuous root-hairs. I might 
cite other cases, such as the fleshy roots of Sonchus arvensis, 
the roots of Taraxacum , of Prunella vulgaris , and very many 
more, but these illustrations are sufficient for my present 
purpose, which is simply to show that these root-hairs aro 
poorly developed if not entirely absent in certain categories of 
plants, and relatively abundant in other plants and under 
other circumstances. On some future occasion I hope to 
enter more fully into this subject, but, at present, I confine 
myself to these statements as preliminary to the record of 
some experiments made by me with a view to the investiga¬ 
tion of the circumstances which give some plants the advantage 
over others when growing together, as in a pasture. 
It is obvious that root development and the power of 
turning to account varying conditions, physical and chemical, 
of the soil, must be important factors in the competition and 
struggle for existence among plants. So far as absorption 
of nutritive matter is concerned it is the minutest 
fibrils and the root-hairs (when present) that we have to 
consider. We know, in a general way, that their number is 
much greater in the same species of plant under some circum¬ 
stances than under others, that if they are growing in rich, 
moist, well aerated and well drained soil, the production of 
feeding roots is greater than under opposite circumstances. 
Under such favorable conditions the root fibrils are short and 
densely matted, whereas in sterile soil they are elongated and 
produce but few fibrils. The dense leash of feeding roots made 
by trees whose roots have access to water is familiar to all wha 
