462 



Marvels of the Universe 



unless he can fall back upon a second line of defence. Here comes in the use of the inner door 

 made by the species of Nemesia above mentioned. Abandoning the outer door, he plunges deeper 

 into the burrow and thrusts up the inner door so as to block the passage. If the wasp forces this, 

 the Spider still has a chance, and retreating up the side passage, pulls the door after him. If the 

 wasp then descends to the bottom of the burrow, and finding it apparently unoccupied, quietly 

 departs, well and good ; but if he detects the inner door, now flush with the wall of the main tube, 

 and attempts to force it, the Spider's last hope lies in the strength with which he can hold it shut. 

 Trap-door Spiders are distributed all over the warmer parts of the world, and are akin to 

 the great Bird-eating Spiders. They vary greatly in complexity of structure, the most highly- 

 organized having the jaws and forelegs armed with rows or clusters of strong spines to help 

 in digging. They live in all kinds of places, some, indeed, building their silk burrows on the 

 surface of tree-trunks ; but to show, in conclusion, that their activities are guided entirely by 



instinct, it may be mentioned 

 that the newly-hatched little 

 ones make trap-door burrows 

 as perfect at the start as those 

 of their parents after months, 

 perhaps years, of practice. 



WHY A LEAF FALLS 



BY E. J. SPITTA, F.R.M.S. 



This curious and wonderful 

 phenomenon is brought about 

 by the growth of an intercept- 

 ing wall of cells which cuts off 

 the flow of the sap from the 

 leaf at the junction of its stalk 

 with the stem of the plant. It 

 is called the " cork " layer. To 

 understand this fully, resort 

 must be had to the examination 

 microscopically of a longitudinal 



1';J 



Photo 6u] 



Here is a slightly magnified photograph 

 Horse Chestnut showing the junction of iht 

 plant. Notice the growth of the cork ceils 



IE., I. SpilK,, F.R.M.S. 



WHY A LEAF FALLS. 



of a section cut along the stem or a 

 leaf-stalUs with the main stem of the 



it the -junction of the left-hand stalk 



with the main stem. scction through the stem of the 



plant at its point of union with the stalk of the leaf. A photograph of this is seen slightly 

 magnified in the illustration on this page. It was made from the Horse Chestnut. 



The stem of the plant has two leaf-stalks attached to it, one on the left and one on the right as 

 looked at in the photograph. It is the right one we first examine. A portion in this situation is 

 shown more magnified in the full-page plate, where it will be seen coming from the lowest part of 

 the stalk (at its junction with the stem) and reaching almost across its entire width, a finger-like 

 projection of cells all piled one on the other, and all closely approximated and packed together. 

 The top or tip of this finger is illustrated at a great magnification in the last illustration, where 

 it is readily seen to be blunt-ended and larger than the part which supports it, as the head of a 

 pin is larger than the pin upon which it is carried. This is supposed to be for the purpose of pushing 

 the cells of the stalk aside, and to make room for the intercepting wall to grow and increase in 

 thickness. 



The sap from the mother tree, of course, usually has free access to the leaf bv passing through 

 the stem and then the stalk ; but this cork layer, which commences to grow in the early autumn 



