INTRODUCTION. 3 



usually similar cells are connected so as to form large groups or masses. An aggre- 

 gation of cells growing in common is termed generally a Tissue (Tela, contextus ; in 

 compound words IutLov^). Each tissue, which is characterised by definite properties, 

 and distinguished from others, is called a tissue-form or, better, a sort of tissue. For 

 the single cells which belong to a tissue, or for each elemental form derived from 

 such a cell, the term tissue-element may be reserved. Tissue-elements which occur 

 singly between dissimilar ones (Idioblasts, after Sachs' terminology) usually cor- 

 respond in their properties with others which occur in connection with similar 

 elements. They are then to be reckoned as of the same sort of tissue as the latter. 

 In like manner, finally, such tissue-elements as occur only as Idioblasts (for instance 

 many laticiferous tubes) will form together with one another a special sort of tissue. 

 All tissue-elements, which correspond in definite similar properties, are therefore 

 termed collectively a sort of tissue, whether they be Idioblasts, or are connected with 

 like elements. 



The course of description which is followed in this book may be gathered from 

 what has been said. The first subject is the characterising and distinguishing of the 

 sorts of tissue, which serve as vegetative organs; then the grouping and arrange- 

 ment of these in building up the members or organs of higher rank. In this course 

 of description a difficulty certainly arises : this can only be overcome by the establish- 

 ment of a limit, which is to a certain extent artificial. Those tissues, which act 

 functionally as vegetative organs, are often continuous in the plants in question -g^ith 

 those higher members, which are according to their most important adaptation 

 reproductive organs. The member of many Ferns which acts functionally as a 

 Prothallium is principally composed of chlorophyll-containing Parenchyma, similar 

 to that of foliage leaves. This sort of tissue, together with vessels, vascular bundles, 

 &c., takes part in the construction of the parts of the flower of many Phanero- 

 gams, &c. Many peculiarities of vegetative tissue, which appear in these parts, depend 

 much less, it is true, on the properties of the single tissue-elements, than on their 

 arrangement. Since these peculiarities are directly connected with adaptation to the 

 generative process, the study of them should also be in connection with it, and must 

 be excluded from this treatise. In cases of sharply-defined phases of development, 

 the boundary, which must needs be drawn, is evident at first sight. For instance, no 

 one will expect the Fern-Prothallium to be treated of here. But among the Pha- 

 nerogams there often occurs a gradual transition between purely vegetative and 

 reproductive organs. To satisfy in the present case the necessity of a definite limit 

 to the subject in question, all that falls under the definition of flowers, specialised 

 inflorescences, or parts of inflorescences, will be excluded from consideration. 



' As has been already intimated, the differentiation of the sorts of tissue is a 

 phenomenon which accompanies the development of a part to maturity. Originally 

 the cells of a part differ, it is true, in certain relations, both in form and direction of 

 .division ; but they correspond in structure, and in the fact that, while they increase 

 slowly in size, they divide repeatedly ; and thus finally produce cells, which develope 

 into tissue-elements. From those phenomena of division, such masses of cells are 

 termed Meristem ; and when they form the first foundation of a member primary 



Compare Unger, Anatoraie, p. 138; Sachs, Textbook, English edition, p. 70, 



B 2 



