44 SCIENCE PRIMERS. [x. 



lengthwise through the blade, meeting at its tip, or 

 one such bundle splits up at the base of the leaf into 

 many that run as above ; in most Monocotyledons 

 these main bundles are connected by straight trans- 

 verse bundles. To these rules there are exceptions, 

 but they are sufficiently constant to make it always 

 worth the while to examine the venation of a leaf 

 together with the characters given in Pars. 39, 53, 

 and 60, when referring a plant to one of these two 

 classes. 



74. The death and separation of the leaf previous 

 to its fall from the parent plant are not accidental, 

 but due to the following causes : 



First, and chiefly, because there is developed at 

 the base of the leaf, or its stalk (if it has one), a 

 transverse layer of cells destined to die after the leaf 

 has performed its functions and before any other 

 part of the leaf does so. The leaf consequently falls 

 off, leaving a clean scar. Secondly, because the leaf 

 rapidly acquires in spring its full size, whilst the branch 

 on which it grows goes on thickening ; consequently, 

 the tissues at the point of union tend to become 

 disunited. Thirdly, because the fluids taken in by 

 the root go to the leaves ; these fluids contain earthy 

 matter, most of which is deposited in the leaf 

 tissues, choking them, preventing them from per- 

 forming their functions, and hastening their death. 

 This is proved by burning spring leaves, which yield 

 little ash, while autumn leaves yield more even than 

 wood does. It is however remarkable that the sub- 

 stances contained in falling leaves are those which 

 have ceased to be of value to the plant. The starch 

 and protoplasmic substances, together with the most 



