4 ORGANOGRAPHY. BOOK I. 



which- it hes, and to which it usually adheres; but occasionally 

 elongates less rapidly, when it is broken into minute portions, 

 and is carried along by the growing membrane. In direction 

 it is variable (Plates I. and II.); sometimes it is straight, 

 and attains a considerable length, as in some fungi; some- 

 times it is short and straight, but hooked at the apex, as in the 

 lining of the antlier of Campanula ; occasionally it is straight, 

 and adheres to the side of membrane, as in the same part 

 in Digitalis piu'purea ; but its most common direction is spiral. 

 Whether it is solid or hollow has not been fully demonstrated ; 

 Purkinje asserts that it is hollow, as will be hereafter men- 

 tioned; but there can be no doubt that it is also, at least 

 sometimes, solid, as in the fibrous bladders of the leaf of Onci- 

 dium altissimum; it is the opinion of many that it is hollow 

 in the case of spiral vessels. I^lementary Fibre has a constant 

 tendency to anastomose, in consequence of which reticulated 

 appearances are frequently found in tissue. Slack adds that 

 it sometimes branches. 



Tlie forms under w^iich the elementary organs are seen are, 

 1. Cellular tissue; 2. Woody tissue; and, 3. Vascular tissue. 



It is almost certain that all these forms are in real it}' modifica- 

 tions of one common type, namely, the simple cell, however 

 different they may be from each other in station, function, or 

 appearance. For, in the first place, we find them all developed 

 in bodies that originally consisted of nothing but cellular tissue ; 

 a seed, for instance, is an aggregation of cells only; after its 

 vital principle has been excited, and it has begun to grow, woody 

 tissues and vessels are generated in abundance. We must, there- 

 fore, eithei- admit that all forms of tissue are developed from the 

 simple cell, and are consequently modifications of it ; or we must 

 suppose, what we have no right to assume, that plants have a 

 power of spontaneously generating woody and vascular tissue 

 in the midst of the cellular. Mirbel has lately reduced the first 

 of these suppositions to very nearly a demonstration ; in a 

 most admirable memoir on the developement of Marchantia he 

 speaks to the following eifect. ' I at first found nothing but a 

 mass of tissue composed of bladders filled with litde green balls. 

 Of these some grew into long slender tubes, pointed at each 



