ISOETES JAPONIC A, A. BK. 



361 



development. A somewhat similar conclusion was arriv^ed at by Yapp (26) as a result 

 of his ecological studies on several morphological features exhibited by Spir(Ba TJlmaria. 

 These strands are differentiated from the mesophyll parenchyma, and normally form 

 four main and two accessory groups of fibre-like cells. Of tbe former, three are situated 

 on the adaxial and only one on the abaxial side of the leaf. The accessory strands occur 

 between the dorsal and corner strands. The main strands are very w^ell developed, but, 

 even in the terrestrial species (e. g. /. Bystrix), the individual fibres are only slightly 

 lignified. Possibly these strands have but little mechanical sii:rnificance : ligrnificatiou 



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of a cell- wall, whilst increasing the rigidity of the cell, decreases its strength. It was 

 found, even on the same plant, that some leaves possessed these strands whilst others 

 did not, and it sometimes happens that only three or four strands are developed. 

 Moreover, individual strands do not necessarily extend along the whole length of tlic 

 leaf, but usually show a tendency to die away or even completely disappear at certain 

 points, especially in the basal region of the leaf. This fact may be correlated with the 

 intercalary growth of the leaf, since the region of meristematic activity is situated near 

 the base of the leaf. 



Towards the margin of the leaf, particularly near the base, the cells become tan- 

 gentially elongated, and their walls are thickened but not lignified. 



4. Vascular Bundle. 



A single collateral vascular bundle with normal orientation occupies an approximately 

 central position in the leaf. The vascular bundle in the adult leaf is surrounded by a 

 layer of large cells, w^hich are fairly compact and regularly arranged. This row of 

 cells probably constitutes the true endodermis, for although the radial walls of these 

 cells are unthickened, yet they are highly refractive and present a more or less wavy 

 appearance. 



Immediately within this layer of cells we find a varying amount of conjunctive 

 parenchyma, amongst which a few schizogenous canals are formed. These canals 

 caueaux aeriferes of Janczewski, 17) arise at a very early stage in the development 

 of the leaf, i. e,, before the differentiation of metaxylem and metaphloem. The walls of 

 the cells bordering these canals are not cuticularized. 



Por our present knowledge of the structure and development of the phloem tissues of 

 the leaf of Isoetes w^e owe much to the excellent work of Kruch (19), who made a 

 comparative study of the conducting tissues of the leaves of several distinct species. 

 "Whereas the development of the xylem tissues varies in a striking manner in different 

 parts of the leaf, the phloem tissues, on the other hand, are well developed throughout 



the whole length of the leaf. 



In the young leaf, the phloem consists of sieve-tubes without companion-cells, and 

 parenchyma. The former are arranged in an arc with its concave side facing the 

 xylem. Protophloem occurs in the central part of the phloem-arc, whilst tbe later- 

 formed sieve-tubes are usually collected together into two lateral groups separated from 

 one another by the few protophloem elements, which, at a later stage, appear as a very 

 narrow layer of crushed cells surrounded by ground-tissue. In the mature leaf, 



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