16 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



leaf-organs organs arising from the end of a shoot-axis are known. 

 They occur in the flowers of many plants, as we shall see later ; the 

 cotyledon in many monocotyledonous plants is terminal in the embryo ; 

 there are also monocotyledonous embryos upon which leaves arise although 

 no vegetative point of the axis is visible, and a similar condition is also 

 found in Isoetes ; further, the vegetative body of Lemna is nothing else 

 than a leaf producing leaves 1 , it is not a leafless twig 2 as is commonly 

 assumed. 



All attempts that have been made to give a simple definition of 

 ' caulome ' and of ' phyllome ' have failed, and this is not surprising seeing 

 that none of the characters upon which they have been based are constant 

 in all of the different cycles of affinity. Plants, it must be remembered, 

 are living things, and the formation of their organs cannot be circum- 

 scribed by definitions. What we can say, and what indeed is alone of 

 interest, is this the modifications which the formation of the organs 

 undergo in any one group can only be determined by comparing all 

 their characters. We have no data enabling us to determine the phylo- 

 genetic history of the leaves of the Spermaphyta, and therefore a fictitious 

 interest only attaches to speculations upon the subject. We shall see 

 hereafter that the part of the plant-bod}- which, with Sachs, we designedly 

 term slioot, has become differentiated into stem and leaf, often in the 

 most different ways in different groups of the plant-kingdom. 



The hair supplies us with a striking illustration of how misleading 

 it is to endeavour to find diagnostic marks of an organ in one character. 

 Hairs or trichomes are structures which are found upon the epidermis 

 of plants. It is doubtful in the case of ordinary typical hairs whether 

 there is any advantage in combining under one name organs which, 

 whilst they share with one another so superficial a character, are yet 

 fitted for the performance of the most different functions. There would 

 be some sound foundation for the terminology if these organs stood 

 in any intimate genetic relationship with each other ; if, for example, 

 it could be proved that the gland-hairs of the Labiatae are the 

 homologues of the woolly hairs which are found in so many species of 

 the family, and that these two hair-forms were derived from a common 

 ancestral form or could be transformed one into the other 3 . Such a con- 

 nexion may be established in many hair-forms, but it certainly does not 

 exist in very many others, and the majority of different hair-forms have 

 their epidermal origin only as a common character, and this is but a 

 superficial one. If I were to find in an intercellular space within one 



1 See my ' Pflanzenbiologische Schilderungen,' ii. p. 274. 



2 Hofmeister described the leaves of Pistia as leafless twigs 



3 I have no doubt that hairs do change their function. In a recent paper I have shown that the 

 water- secreting hairs of Rhinantheae arise from gland-hairs. See Flora, 1897, p. 444. 



