10 LECTURE I. 



unicellular plant which has a particular function to discharge ; 

 thus the cilia of the zoospore of Hsematococcus are its motile 

 organs. 



In these plants, in which, as we have seen, the physiologi- 

 cal division of labour finds its fullest expression, there is 

 necessarily a mutual dependence between the various organs ; 

 no one organ can discharge its function unless the others 

 discharge theirs in an adequate manner. It is important 

 therefore that there should be some means by which the 

 organs, which are often widely separated, may be placed in 

 direct communication with each other, and we find accordingly 

 that certain cells are especially adapted for this purpose, such 

 as the laticiferous cells and vessels which are present in many 

 plants, and the cells constituting the fibre-vascular tissue, and 

 it is further effected by the intercellular spaces. 



We will now briefly consider the mode in which these 

 different kinds of cells are developed. In these differentiated 

 multicellular plants the formation of new cells is confined to 

 certain definite regions, and at first the cells are very similar 

 to each other. A group of cells in this stage corresponds to 

 an undifferentiated multicellular plant, but important differ- 

 ences soon make themselves apparent. In the first place, it 

 becomes evident that the growth of each of the constituent 

 cells of the group does not proceed independently, but that it 

 exhibits a certain correlation with that of the others ; such a 

 group of cells we term a tissue. Secondly, the cells in their 

 growth come to differ more or less widely from each other, 

 the form assumed by each cell bearing a definite relation to 

 the function which it is destined to perform in the economy of 

 the plant. Of the various forms which the cells assume, those 

 which resemble each other are connected together, so that 

 several tissue-systems can be distinguished in the body of the 

 plant. The plant is then said to exhibit differentiation of 

 tissues or histological differentiation, and this is the expression 

 of the adaptation in different directions which the originally 

 similar cells have undergone for the due performance of the 

 various functions of the plant. 



It is in 'the multicellular plants also that the highest 



