A Text - Book of Botany (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1912 Excerpt: ...play no great part in nature they need not be further considered. Those directive movements which are brought about by the unequal distribution of dissolved or gaseous substances in the neighbourhood of the plant are of much greater importance; these movements are termed chemotactic and chemotropic. Chemotropism (102). In the case of Fungi and of pollen-tubes chemotropic movements have been demonstrated which bring the organism into a certain concentration of particular substances; this, concentration is the optimal one. With the same organism and the same stimulating substance these movements are sometimes positive and sometimes negative; positive when they lead towards a higher concentration of the substance, and negative in the converse case. In the case of pollen-tubes sugar is the chief substance that acts as a stimulus; in fungi, in addition to sugar, peptone, asparagin, compounds of ammonia and phosphates. There are also substances such as free acids which always have a repellent influence even in extremely weak concentration. Chemotropic irritability has also been demonstrated in roots, though it cannot be said that it plays an important role in their life. In the examples of chemotropism given above the stimulating substances were solid substances in solution. When on the other hand the plant is induced to perform directive movements by the unequal distribution in a space of aqueous vapour or gases, a distinct name has been required, though no distinction of principle can be drawn. Irritable movements caused by differences in moisture are termed hydrotropic (103), while those brought about by gaseous differences are termed aerotropic (104). Aerotropism has been proved for pollen tubes, roots, and shoots, and hydrotropism for roots and moulds. Thus ...